Considering the unsteady aerodynamic characteristics of the vertical axis wind turbine (VAWT) blade at low speed ratios, the B-L dynamic stall model was modified. The finite aspect ratio effects and blade tip loss were taken into account to modify the aerodynamic characteristics for constant chord blade. Based on the double-multiple stream tube method (DMST) in aerodynamic analysis, an improved aerodynamic performance prediction model for VAWT was developed combining with the dynamic stall model and predicting method for the effects of finite aspect ratio. The aerodynamic performance of 17m VAWT by Sandia National Laboratory was calculated. The effects of the dynamic stall and finite aspect ratios were analyzed, and the rotor power and power coefficients were compared and analyzed between the test and evaluation results. Feasibility of the prediction model to evaluate the aerodynamic performance for VAWT was proved.