Abstract: To investigate the flowfiled characteristics of airfoil dynamic stall, the advanced 3D-PIV technology was employed to measure the dynamic stall flowfield of helicopter rotor airfoil SC1095. It was found that the leading edge vortex convective velocity changed with the location of vortex. The dimensionless vortex velocity was 0.39 when the vortex was moving on airfoil surface, and was 0.55 at airfoil wake zone. Based on the different leading-edge vortex convective velocities, classical Leishman-Beddoes model was then modified, in which the vortex time constant in classical model is replaced by a time varying function. Comparing with the classical model, the peak of airfoil normal force coefficient calculated by the modified model was increased about 5%, and the negative peak of airfoil moment coefficient was promoted about 13%; both of these had better agreements with the experimental data, and it was demonstrated that the modified model gave a more reasonable physical meaning of airfoil leading edge vortex.