A rate equation model is established to describe the threshold characteristics of a diode-pumped Cs vapor laser in this paper. With the help of the numerical solution for the model, the influences of several parameters (waist position and waist radius of pump laser, length of vapor cell, and operating temperature) on the threshold pump power are analyzed. Results show that there are optimal values of the pump waist position and radius, vapor cell length and operating temperature to ensure a lowest threshold pump power. Besides, the decrease of divergence angle can reduce threshold pump power effectively. Simulation results are in good agreement with the experimental data. It is shown that the model can be used to reveal the threshold characteristics of diode-pumped alkali vapor laser and may serve as a theoretical guidance for the optimization design of this laser.