This article reports a simple theoretical model developed for the temporal performance of a short laser pulse pumped, gain-switched, four-level laser system. The model is based on the rate-eqations of four-level systems and some practical physics conditions. From the model, we obtain some analytical relationships between the build-up time, pulse width of the output laser pulse and the pumping energy and energy density. The rationality of the model is analyzed according to the expermental results.