For the coherent optical recombination of high power HF chemical laser beams such as master and slave oscillator array (MASOA) or coherent optical adaptive technique (COAT), a wide bandwidth servo system is required to control the phases of the laser beams. A phase-control servo system using a rate feedback and a laser Doppler sensor was designed. Its performance was simulated on a CDC7600 computer. Laser frequency drift was modeled by a Gaussian Markov random process and the mechanical response of the PZT mirror-driver was modeled by a transfer function. The result indicated that a rms steady-state phase error of less than 3 deg. can be achieved. More detailed discussion on the servo system design and simulation will be given.