For investigating the aerodynamic characteristics of rigid helicopter rotor under high speed flight speeds, the complex flow field around helicopter rotors are numerically simulated by solving the three-dimensional unsteady Reynolds-averaged Navier-Stokes (RANS) equations based on multi-block structured finite volume method (FVM). Here both hovering and forward flight conditions are considered. In the research, the effects of dynamic flow separation, local spanwise /radial flow and relevant factors are emphasized. On one hand, we focus on the influence of collective pitch of rotors on aerodynamic loadings as well as unsteady reversed flow of rotor blades during the withdrawal stage; On the other hand, distinctive laws of aerodynamic forces on blades under both hovering and forward flight are revealed. Numerical results show that, the lift over a rotor increases linearly with its collective pitch. However, during a forward flight with large advancing ratio(AR), reversed flow will appear at the blade root area in their withdrawal stages, and these produce totally different pressure distribution compared with conventional ones, which causes the lift mainly coming from the advancing blades. The numerically results show satisfactory agreement with the data from wind tunnel experiments.