Factors affecting diffraction of microlens array are analyzed, distribution expression of optical field intensity is deducted on the focal plane of microlens, and the physical explanation about Fresnel number used for evaluating diffraction effect is given. Moreover, simulation of microlens array is carried out by ZEMAX software, and the distribution of optical intensity on the focal plane is calculated. At last, comparing calculating results under different conditions, the rationality of Fresnel number as validation criteria of diffraction effect and the feasibility of Fresnel number used to judge disturbance among focal spots are validated at the same time.