Based on the Franz-Nodvik mode, a detailed numerical analysis of the contributions of the fluence of the pumping light and signal light as well as the crystal parameter to the output pulse energy and conversion efficiency is presented. Comparison between the theoretical result and the experimental result shows that the theory is quite valid. The calculation indicates that in the system of chirped pulse amplification, there is an optimum fluence of pumping light. Reasonably choosing the fluence of pump light will not only advance the conversion efficiency effectively, but also remarkably reduce the demand of signal light energy and the output instability caused by the fluctuation of signal light energy.