Based on Rayleigh-Sommerfeld scalar diffraction formula, analyzed is the diffraction field distribution of the restricted output end surface of multiple quantum wells planar waveguide by slit. Obtained is its analytical expression of field distribution, which permits accurate and effective study on the characteristic of diffraction field from the restricted output end surface of the waveguide by slit. Then, the variation curve of the beam propagation factor M2 versus the slit width is computed by the second moment method. It is useful for understanding the restricted diffraction properties of the multiple quantum wells planar waveguide. When the slit half width is bigger than the core layers half width, the beam propagation factor M2 value tends to a constant 1.108. Therefore, the corresponding field amplitude distribution is approximated by Gaussian function, and the far field divergence half angle(θ0,G=0.091 8) is calculated by matching efficiency method.