Based on the mathematical model of photovoltaics and the concept of average model, average-state mathematic modeling was done with a Buck circuit. Based on the sliding-mode theory and operating characteristic of photovoltaics, a fixed-frequency-sliding-mode (FFSM) controller was proposed for photovoltaic system MPPT with the existence and the stability proved. Through sufficient theoretical analysis, system simulations and experiments were implemented .The results of simulations and experiments show that the fixed-frequency-sliding-mode controller can achieve photovoltaic system MPPT with tiny buffeting and the control arithmetic works well.