In the usage of the knife-edge method, the influences of the measurement errors in the position and the light spot width on the measurement errors in the beam quality factor M2, the far field divergence angle, the beam waist radius, and the light waist position are theoretically investigated. In addition, based on the beam quality analysis instrument, the theoretical results are tested. The results show that the influence of the measurement error in the light spot width on the laser beam quality is much stronger than that in the longitudinal position.