Abstract: As the main lift and control component of helicopter, rotor has complex flow field structure, such as unsteadiness, blade-vortex interaction and tip vortex. It is difficult to study the rotor flow field. To solve this problem, an experimental study of rotor flow field in hovering state was carried out by combining phase lock technology and particle image velocimetry(PIV) technology. Then the main energetic modes were extracted by proper orthogonal decomposition(POD) to describe the space-time evolution of the flow field. The results show that the rotor wake is presented as an inverted triangle structure in 2D structure, so that is an inverted cone structure in 3D structure. According to POD energetic analysis, blade vortex structure contributes the most to turbulent kinetic energy in rotor wake, followed by tip vortex structure.