Compared with the traditional rotational method, a method for the focal length measurement of microlens array based on grating diffraction has a higher efficiency, because this method need not rotate the collimator. As microlens array has a large number of cells generally, the disturbance diffracted from nearby cells also needs analysis. The interferences between 0-order and ±1-order diffracted focula of grating are analyzed by Matlab. The proper wavelength and grating can be determined for different kinds of microlens array by analyzing the wavelength, grating period, sub-aperture and focal length of microlens array. The disturbance can be avoided and the focal length measuring of microlens array can be finished. The experimental results show that this method is fit for the testing of microlens array with small F-number and many cells because of its high precision and efficiency.