According to the international standard of laser beam quality, the beam quality of coherent array laser is evaluated by the beam quality factor [M2]. And the [M2] of coherent array laser is calculated by fitting the hyperbolic equation of coherent array laser transmission under different wavelength, sub-source radius, duty cycle and array width. Considering that there is a large error in calculating the [M2] factor by fitting hyperbolic equation, the beam quality of coherent array laser is calculated by using power ratio in bucket (BQ). According to the relationship between BQ and [M2], the stable [M2] of coherent array laser under different parameters can be obtained by calculating BQ. The calculated results show that the beam quality of coherent array laser is close to the diffraction limit.