Beam quality M2 factor of the single-aperture coherent laser beam synthesis system is a major problem to be solved. Based on the definition of second-order moment, beam quality M2 factors of the coherently combined beam supposed by the basic model TEM00, TEM01and TEM10 are presented by some analytical expressions in this paper. And, the effects produced by the beam waist width, the propagation distance, the ratio of the two amplitudes and the distance between the two coherent sources on the beam quality M2 factor are numerically analyzed. Some results show that the beam quality M2 factor of the coherently combined beam is unchanged when the propagation distance parameter d1100. And the experiment is carried to verify some theories of the two TEM00 coherently combined beam.