In this paper, in order to improve the transmission capability of contactless power transfer (CPT) system and overcome the problem of system resonant frequency changed in a dynamic load switch period, take the topological structure of serial-parallel CPT system for example, first the optimal values of the compensated capacitors are determined. Then designed a primary side control method based on single chip microcomputer. On the basis of that, optimize the system parameters, to make the system run more stable in a dynamic load switch period, and the transmission power has been improved. At last it was proved by simulation and experiment.