We propose a mathematical model for the phase-locking of a mode-locked laser. Based on the standard phase-locked loop (PLL) theory, a detailed mathematical model is developed. By applying the model to a system of phase-locking of a mode-locked laser, we analyze its three basic characteristics: the final steady error, the transient response, and the stability. An experimental phase-locking system is built with the standard PLL to measure the actual locking performance. The agreement between the experimental results and simulations has proved that our theoretical model for phase-locking of a mode-locked laser is correct.