In this study, to solve the problem of interference of the pulse signal with the dither, which makes the Mach-Zehnder electro-optic modulator (MZM) operating point unstable, a dither-adaptive bias-control technique is proposed. A digital control system was used to generate a dither with variable frequency. The MZM can be locked at the minimum operating point of the transmission curve by the harmonic components of the feedback signal analyzed by fast Fourier transform. By the real-time monitoring of the output bias voltage, the dither frequency is able to automatically change, thus avoiding the influence of the pulse signal. Accordingly, laser pulse modulation with high extinction ratio is realized. Experiment results show that the pulse signal will affect the stability of the modulator operating point when the laser pulse frequency is close to the first harmonic frequency of the dither. The system can realize adaptive change of the dither frequency in the range of 0.45--2 kHz to meet the requirements of laser pulse modulation at different repetition frequencies. When the peak value of the pulse is 10 V and the duty cycle is 10%, the extinction ratio of the modulated optical pulse can reach 26 dB, which is approximately 24 dB higher than that when the pulse signal interferes with the dither.