A passively Q-switched Nd:YAG laser pumped by diode pulse is set up. The characteristics of the laser pulse intervals are analysed by combining numerical simulation and experimental method. Starting with the rate equation of passively Q-switching mode, the Q-switched pulse intervals are formulated in consideration of the feature of diode pulse pumping and the influence of surplus inverse population density. The in fluence of pumping current and pulse width, the reflectivity of output mirror and the initial transmission of Cr4+:YAG on pulse interval time is analysed enphatically. The result indicates that the numerical simulation of pulse interval time coincides with the experimental data.