We investigate the optimization of Q-switched lasers based on maximizing the laser peak power and apply the Lagrange multiplier technique to obtain the numerical solutions of the output mirror reflectivity and several key laser parameters such as the peak power and output energy. Meanwhile, according to the optimization theory described in our paper, a Nd:YAG Q-switched laser is studied experimentally and the results showed this theory is very useful in high gain-to-loss ratio laser design tradeofls.