In the design of low-power mode switching power supply, reducing power loss so as to improve the efficiency of power supply is one of the most important goals, and the flyback transformer design is particularly critical. In this paper, the author optimized the design of the flyback transformer and made a theoretical calculation, and obtained a set of optimized design parameters. Then the author confirmed the effectiveness of optimization through SABER simulation. The results show that power loss has been markedly reduced after optimization.