The optimal effectiveness model for optimizing the system parameters was established in this study by combining the optimization function based on the pre-laser Q-switching technique. By introducing boundary conditions to solve the rate equations, the specific process of composite mode selection is analyzed, and the specific pre-laser modulation time interval that enables single-longitudinal-mode laser output is obtained. Furthermore, through the established efficiency evaluation function for single-longitudinal-mode laser output characteristics, with the maximization of single-pulse energy as the objective, an energy efficiency analysis of relevant parameters in the mode selection components is conducted. The model we have established is universal, which can accurately control the pre-laser Q-switching process and reveals the inherent logical relationship of the relevant parameters in composite mode selection. The output characteristics of a single-longitudinal-mode Q-switching laser can be predicted and adjusted under the condition of high pump injection. Both simulation and experimental results confirm the effectiveness of the model.