Based on OCT, OCTA is a newly invented, non-invasive, fast and safe technology of volumetric imaging about blood flow. According to the OCT signal information they use, methods of OCTA can be divided into three categories: phase based algorithms, amplitude based algorithms and complex algorithms. After depicting the imaging principle, specific algorithm and advantages/disadvantages, methods of these three categories are compared in which complex category achieve best performance on imaging. Research progress is combined to discuss the prospect of OCTA.