As a kind of high-quality factor thermo-electromechanical coupling device composed of isotropic elastic-optical crystal and piezoelectric crystal, photoelastic modulator (PEM) is widely applied for polarization measurement, spectrum measurement, and many other purposes. However, the resonant frequency tends to drift with temperature changes in the high-voltage resonant state, which destabilizes the phase modulation amplitude of the photoelastic modulator and reduces the driving efficiency. To solve this problem, the resonant frequency characteristics of the photoelastic modulator are analyzed at first. Then, a compound resonant network model of the photoelastic modulator and its high voltage resonant driving circuit is established, and a solution to frequency tracking based on the amplitude-frequency characteristics of the resonant network is proposed. Besides, a control test system based on field programmable gate array (FPGA) is developed to achieve resonant frequency tracking and modulation amplitude measurement. The test results show that this method is applicable to track the resonant frequency effectively and improve the stability and driving efficiency of the elastic light modulator. The duration of the test exceeds 90 min, and the standard deviation of the phase modulation amplitude is 0.83% rad.