In certain conditions，magnetic pendulum periodically driven by a coil can become phase-locked with the driving signal and lead to stable oscillation. We study the transient process of the phase locking and measure the amplitude and phase of the pendulum in various controlling conditions. Based on the nonlinearity of 　　pendulum and the phase-dependent energy transfer between the pendulum and driving coil，a negative feedback mechanism is identified in the system，from which all the experimental results can be explained quantitatively.