The unsteady calculational method of secondary air system in gas turbines was studied. The whole system was divided into several length correlated parts and discrete loss parts. The method of characteristic line was adopted for single length correlated parts

while partial differential equations of continuity

momentum and energy were transformed to ordinary differential equations under specified conditions

so the interaction of flow and heat transfer inside the parts itself could be taken into account. When the method of flow network based on pressure correction was used for discrete losses parts

momentum equations were solved prior to the calculation of respective energy equation. The process of data transfer on the interfaces between the mentioned two types of parts was discussed. With the method of time step definition and improved analytical models for different parts adopted

unsteady analysis of secondary air system was achieved. The program was validated against steady experimental data of the whole system and unsteady calculational data of individual parts; in 10s' calculational process

the result achieved by the program deviates 10.2% from that by CFD. The results indicate that the program can satisfy the need of unsteady calculation on secondary air system.