The theoretical model of quasi-three-level system was modified through considering different lifetime of laser upper and lower level and spontaneous radiation from upper level to lower level. Numerical simulation was carried out and the law of 946 nm Nd:YAG laser performance varying with system parameters was obtained. At a certain doping concentration and output loss, there is an optimum crystal length and an optimum ratio of pump and laser beam radii. The experimental results accord with theory analysis.