To solve the problem of incomplete penetration and the welding torch hitting the plate during thewelding process of Q345B large thick plate by using robot double-sided arc welding，the method of using no reserved-clearance root was proposed. The good forming weld joint was obtained by increasing the welding line energy and reducing the arc distance. The microstructure and mechanical properties of the welding joint were investigated according to the AWS D1. 1. The results show that the first weld pass of root consists of acicular ferrite and bainite，and the second weld pass of root and filling pass consist of acicular ferrite and few of proeutectoid ferrite，and the microstructure of fusion region consists of acicular ferrite and lath martensite，and the coarse grain region consists of larger lath martensite . The average tensile strength of Welding joint reaches 536. 47 MPa，and the tensile strength of welding is larger than fusion. Impact property of welding joint is good，and the highest hardness reaches 281 HV.