A method for determining the five-parameter model of photovoltaic cells is presented. In the method, the four equations expressing relationships among five parameters at the short-circuit point, the open circuit point and the maximum power point of I-V characteristic equation are obtained. The effects of diode ideal factor (a) on output characteristic of photovoltaic cell are analyzed. And then another equation is constructed by using data of normal condition provided by the manufacturer to obtain the values of five parameters, based on which the parameters of photovoltaic cell under any radiation and temperature can be solved. So the output current and voltage of photovoltaic cells can be solved by using explicit I-V equation based on Lambert W function. Finally, the proposed method is validated to be correct by contrasting with the I-V characteristic and the measured data of maximum power of Shell S65 and TDB125×125-72-P.