Beam quality of the passive coherent beam combination system is studied using the ring cavity passive phase-locking experimental platform of four fiber lasers in two-dimensional distribution. With the times-diffraction-limited factor β as the evaluation method, the impact of duty ratio of sub-beams and different phase-locked patterns on the β factor is analyzed theoretically; and the β factor corresponding to different output power levels is measured in the experiment. The results show that although the β factor can reflect the difference in beam quality under different experimental conditions to some extent, it can not reflect the characteristics of coherent beam patterns in the far-field because the focus of the β factor measurement is to determine the bucket size of 83.8% encircled energy (power) ratio at the focal plane, therefore it is not suitable for the evaluation of the beam quality of a passive coherent beam combination system. A comparative analysis of the β factor and beam quality is done, beam quality can better reflect the characteristics of the coherent beam combination.