An optimized Binary Optical Microlens(BOML) is designed using phase balance design method, this is an improvement on the design of Dammann′s step zone plate. The accumulating integral method is used to analyse the BOML′s diffractive result, and compared with Dammann′s zone plate. The calculated results demonstrate that the efficiency and image quality of this BOML are better than Dammann′s. The advantage of this method is that it can design the BOML with large D/F and arbitrary object and image distance.