This brief studies the effect of the control loop of the interfacing converter on the characteristics of the photovoltaic (PV) cells systematically under the peak-current-mode (PCM) control. First, based on the terminal characteristics of solar cells, the Buck converter with an additional input capacitor is constructed under the PCM control and its small signal model of PV power generation system is built up under the control operating in continuous conduction mode. Then, the stable working region of PV cells under PCM control is deduced according to the essential condition of the Hurwitz criterion. In addition, the parameters of main circuit and control circuit of the system are optimally designed. Finally, the correctness of proposed method is verified by a Saber circuit.