In the present paper a Nd:YAG crystal pumped with a high power fiber-coupled diode laser module at 808nm is optimized by using the simulation of the LASCAD software. The module is used to investigate the optimum cavity design parameters. In the present paper was optimized the cavity design parameters by controlling the cavity length, the beam overlap parameter, the Nd:YAG rod dimension, and the output coupler reflectvity. The practical example of a Nd:YAG laser pumped by a 7.8W fiber-coupled laser diode module is considered to confirm the suggestes physical analysis. The cavity optimum configuration was cavity length 65mm, Nd:YAG rod dimension ø3mmx10mm with 1% doping concentration (1at.%), and output coupler reflectivity 85%. The pump waist spot size wg equal to the beam waist spot size wg . The final optical to optical efficiency is 39%.