In order to further improve the light energy collected by the photo-sensitive area, a new design method for the aspheric microlens fabricated by the photoresist reflow method is proposed, which combines finite element ray tracing with the Lagrange multiplier method. Given the focal length, the needed aspheric surfaces can be computed automatically by the proposed method. In order to verify the accuracy of this algorithm, the RMS radius of the focal spot of the microlens with circular aperture is simulated and theoretically calculated, respectively, and both results are consistent. The approximation error of spot diagram of the cubic finite element ray tracing is estimated. The enclosed energy collected by the photo-sensitive area of 320 pixel×256 pixel ultraviolet photodetectors is computed and the efficiencies of microlens with square aperture shapes and different fillet radii are compared. The results show that the square aperture microlens with a 6 μm fillet radius has a relatively high energy concentration with an efficiency of 96%.