We use an electro-optical potential phase modulator to modulate the pump light to obtain radio frequency modulation transfer spectroscopy (MTS), and study the optimization problem of the zero-crossing slope of the center of the dispersive signal of the MTS spectrum. By changing the modulation frequency of the pump light, the spot size of the pump light and the probe light, we study the parameter dependence between the zero-crossing slope of the MTS spectral signal and the modulation frequency, and spot size. The optimal MTS spectral signal is obtained when the pump light modulation frequency is -3.6 MHz (about 0.69 times the natural linewidth). Finally, by using the optimal MTS spectrum, the DL Pro @ 852 nm laser frequency is locked to the cesium atom D2 line (F = 4) - (F = 5') cycle transition, and the laser frequency fluctuation is about 170 kHz in the 60 minutes sampling time, which is significantly improved compared with the frequency fluctuation of the laser -11 MHz during free running.