The existence of critical angle seriously limits the range of microlens array′s numerical aperture and the improvement of the quality of the microlens in the fabrication of microlens array by using heat-forming photoresist method. Based on the study of the critical angle effect, a new method named step heat-forming photoresist method to expand the N.A. range of microlens array is presents. The results show that this method can effectively improve the conventional heat-forming fabrication. The available diameter range of microlens is from 50 μm up to 900 μm. A series of MLA with F/1 to F/10 relative apertures have been fabricated, with the improved quality of the profile.