The working stability of an optically-addressed phase-only spatial light modulator is analyzed and optimized based on the equivalent circuit simulation of the optically-addressed spatial light modulator and an experiment to measure its stability. The results demonstrate that the voltage waveform on the liquid crystal layer of the light valve causes the phase fluctuation of the readout light modulated by the optically-addressed spatial light modulator and that the driving voltage frequency and write light intensity simultaneously affect the modulator's phase fluctuation amplitude and phase change capability. Based on the aforementioned analysis and by testing the phase change curves of the optically-addressed spatial light modulator, it is found that when the driving voltage period is 95% less than the response time, the phase fluctuation rate corresponding to the maximum phase change of the phase modulation curve is reduced to 0.35%; however, the phase modulation capability is only 0.8λ. By optimizing the driving condition parameters of the optically-addressed spatial light modulator, a phase modulation capability of 1λ is obtained, and the phase fluctuation rate corresponding to the maximum phase change of the phase modulation curve is 1%.