In order to solve the problem of low conversion efficiency of conventional terahertz sources, this paper proposes a cascade differential frequency theoretical model based on periodically polarized potassium titanium phosphate (PPKTP) planar waveguide, and establishes a theoretical design method for realizing high-efficiency terahertz radiation by combining the Cherenkov radiation effect with the Quasi-Phase-Matching(QPM) interaction. Numerical simulation results show that the 15th-order Cherenkov-type QPM cascade differential frequency generation of terahertz significantly breaks through the conventional Manley-Rowe efficiency limit, and the terahertz photon conversion efficiency reaches 424.5%, which is 15 times higher than the cascade-free scheme. By theoretically analyzing the cascade difference frequency combined with the Cherenkov effect to generate terahertz waves, and optimizing the waveguide structure parameters and pump light frequency, the terahertz output power is further improved, providing a theoretical basis for the development of high-power integrated terahertz sources.