A new fuzzy-PI control method was proposed to deal with load fluctuation in in Buck circuit,in which the initial value of PI controller came from the transfer function of the Buck.Also the parameters of PI control and fuzzy variable universes were optimized in the same way.When there is a load fluctuation,the output voltage could stay the same almost with tiny steady-state error and good robust,which make great significance to the converter.Then the method was proved to be correct by its simulation.