Distributed ultrasonic excitation is important for structural health monitoring and nondestructive testing， especially when several detection positions need to be covered by one compact and flexible excitation system. Traditional ultrasonic excitation is mainly realized by piezoelectric transducers， which usually require electrical wiring and local driving circuits at each point. This increases system complexity and limits application in narrow spaces or electromagnetic-interference environments. Fiber-optic laser ultrasound provides a possible solution because the optical fiber can transmit light and also act as the carrier of the excitation unit. However， most reported fiber-optic laser ultrasonic transmitters are still based on single-point excitation， and some multipoint structures involve complex fiber processing or poor controllability of optical power distribution. To solve these problems， this paper proposes a multipoint laser ultrasonic transmitter based on a thin-core fiber–multimode fiber–thin-core fiber structure， namely the TMT structure， aiming to realize controllable cladding optical coupling and balanced multipoint ultrasonic excitation along a single fiber.The TMT structure consists of an input thin-core fiber， a short multimode fiber， and an output thin-core fiber. Owing to the mode-field mismatch between the thin-core fiber and the multimode fiber， part of the core light is coupled into higher-order modes and cladding-related modes when light enters the multimode fiber. After propagation and redistribution in the multimode fiber， part of the optical energy is coupled into the cladding region of the output thin-core fiber. A photoacoustic conversion layer is coated on the fiber sidewall， so that the cladding optical energy can be absorbed and converted into transient thermal expansion， thereby generating ultrasonic waves. BeamPROP simulation based on the beam propagation method was used to analyze the optical transmission and mode coupling behavior. The effects of multimode fiber length and core diameter on the cladding optical power ratio were investigated. Then， TMT structures with different effective multimode fiber lengths were fabricated by micrometer displacement platform-assisted cutting and repeated fusion splicing. The cladding optical power ratio was measured through input and output optical power， and a three-point TMT laser ultrasonic excitation system was built to verify the optical energy distribution and ultrasonic output consistency.The simulation results show that the cladding optical power ratio of the TMT structure can be effectively adjusted by changing the effective length of the multimode fiber. Compared with the structure using a conventional single-mode fiber， the thin-core-fiber-based TMT structure obtains a higher cladding optical power ratio and is more suitable for sidewall laser ultrasonic excitation. Among the investigated multimode fibers， the 105 μm core diameter provides better coupling stability and more favorable cladding energy distribution. For the fabricated three-point TMT structure， the measured cladding optical power ratios of the three excitation units are 34.23%， 32.19%， and 32.43%， respectively， indicating that relatively balanced optical power distribution is achieved. After the photoacoustic conversion layer is prepared， stable ultrasonic signals are obtained from all three excitation units. The peak-to-peak voltages of Sample-1， Sample-2， and Sample-3 are 1146.79 mV， 1129.22 mV， and 1104.37 mV， respectively. The small amplitude difference indicates good output consistency. Frequency-domain analysis shows that the generated ultrasonic signals have a -3 dB bandwidth of about 1.48 MHz and a central frequency of about 5.4 MHz.In conclusion， the proposed TMT structure realizes controllable cladding optical power coupling through the mode mismatch between thin-core fiber and multimode fiber， without complicated special-shaped fiber processing or additional electrical excitation units. The effective length of the multimode fiber provides a practical parameter for controlling the cladding optical power ratio， which supports the fabrication of multipoint transmitters with balanced energy distribution. The three-point experiment verifies that the proposed structure has good optical power distribution consistency and ultrasonic output consistency. Therefore， this multipoint fiber-optic laser ultrasonic transmitter has the advantages of simple fabrication， controllable parameters， good repeatability， and potential scalability， and can provide a feasible excitation scheme for distributed ultrasonic testing， structural health monitoring， and fiber-based nondestructive evaluation.