It is an effective method to obtain high beam quality and high power laser output by coherently com.bining each sub-laser source of the laser array. According to the international standard of beam quality, the bucketradius of coherent array laser is calculated by the energy or power ratio in the bucket. And the beam quality factor M2 of coherent array laser is calculated by fitting the hyperbolic equation of laser beam transmission. The results of calculation show that the beam quality of coherent array laser is close to that of fundamental mode Gaussian beam.