By Fraunhofer diffration theory and numerical analysis method, the relation of beam separation distance, fill factor, phase change and far-field intensity distribution of laser coherent combination is calculated. The influences of transformation characteristic of phase and polarization on intensity distribution of coherent combination are studied by heterodyne method and liquid phase modulator. The characteristics of phase change for beam in fiber transmission is obtained. As a result of the random change of phase, the intensity of laser combination is reduced and the excursion of background comes forth. The contrast of fringes can be changed with the transformation of polarization. In vibrant and temperature changed environment, the frequency for phase change in beam is higher than that in natural environment.