A practical method based on virtual instrument technique in order to realize operating points automatic control for Mach-Zehnder (MZ) electro-optic intensity modulators (EOIM) is propsed. Operating point at the minimum of the transmission curve can be automatically controlled steadily by means of “sweep—phase discrimination and compensation” in a closed loop feedback system. When the output power of the laser is 6 mW, EOIM is controlled steadily at minimum point for an hour. The average optical output power of the EOIM is 1.198 μW, standard deviation is 0.123 μW and fluctuation is less than 0.43 μW at minimum point with control. When the additional electronic pulse signal is modulated at minimum point of EOIM, light pulse with mean extinction ratio (ER) of 25.71 dB is produced. Under pulse modulation, the output pulse ER is 9.94 dB higher than that in the condition without control, and fluctuation is less than 0.5 dB/h. Which improves working stability of MZ electro-optic modulator greatly.