Based on Fresnel diffraction integral formulation,the expression of diffractive field distribution for the laser beam passing through laser scanning components was derived.The amplitude distribution of diffractive field,the spot radius on the scanning surface and the optical resolution of laser scanning system were analyzed numerically.The results show that the resolution parameters like line resolution,light spot number resolution and resolution capability of laser scanning system were affected by the change of waist radius of the incident beam.The sectional function expression of line resolution were given by the approximate and fitting method in different diffraction situation,and error analysis proves its accuracy.