Solid state transformer is one of the key technologies of energy internet, which is closely related to the acceptance of renewable energy and flexible power conversion. In order to solve the problem of low efficiency in high voltage intelligent solid state transformer, focusing on the input stage of the intelligent solid state transformer, the synchronous modulation usually used by the cascade input stage of solid state transformer is improved. The hybrid modulation is adapted to improve the efficiency of the input stage. Finally the efficiency of two modulations is compared based on MATLAB/Simulink simulation, thus the efficiency of hybrid modulation is verified.