To provide full panoramic video content, the transmission bandwidth and playing complexity of virtual reality (VR) video are much higher than that of traditional video. Dynamic stream switching is an efficient scheme which stores multiple asymmetric projections of panoramic video in server side, and transmits only one asymmetric projection (with much lower resolution than full panoramic video) according to the direction of user's head. In this paper, we propose an asymmetric circular (ASC) projection method to generate the asymmetric projection of panoramic video. ASC uses equal area projection in the main viewpoint area to ensure the high and uniform quality of this area, and a projection with decreasing sampling density in other areas. Experimental results show that the bit rate and playing complexity of VR video are greatly reduced with the proposed ASC method. ASC is superior to state-of-the-art method and has been adopted in the standard of IEEE 1857.9-Immersive Visual Content Coding.