Three Raman pulses are usually adopted to make cold atomic beams to split, reflect and recombine when doing cold atom interferomertry experiments. The stability of Raman laser power is as important as frequency, because it causes the Rabi ratio to change with it. When the Rabi ratio changes, the final phase difference between the two separated atom beams changes as well. It is a great disturbance for interferomertry signal stability and also the signal to noise ratio. The effect is more than acceptable when it comes to atom interferometer used as high precision acceleration measurements. Keeping laser power stable can improve phase noise a great deal. Thus a digital control system was designed for higher laser power stability using an acousto-optical modulator(AOM) as a controller, and the feedbacks from the AOM were applied to compensate the power variation outside the cavity of a laser. After realizing the identification with the AOM and a simulation model for it based on the identification results, an PID controller was programmed in the Labview language. Experimental results show that the the laser power instability can be reduced from 0.19% to 1.67% with the method under the measurement time of 1 h. And it works really well for the cold atom interferometer system.