In order to reveal the mechanism of secondary peak of aerodynamic forces of airfoil under dynamic stall， a numerical method for simulating the flowfield of pitching airfoil is established based on moving embedded grid technology， finite volume method， LU-SGS implicit scheme and Roe-MUSCL scheme. Firstly， the flowfield of NACA0012 airfoil under deep dynamic stall is simulated， and the results are in good agreement with test data， which proves the accuracy of the established CFD method. Then， the mechanism of secondary peak of aerodynamic forces is explored by analyzing the flowfield of NACA0012 airfoil under dynamic stall. Finally， the influences of airfoil geometry on the secondary peak of aerodynamic forces are carried out， including the maximum thickness， camber and location of maximum camber. The results indicate that the trailing-edge vortex induced by dynamic stall vortex is the key factor leading to the second peak of aerodynamic forces. The change of the airfoil geometry parameters could cause the dynamic stall vortex and trailing edge vortex to change during the dynamic stall process， which makes the increment of secondary peak relative to the valley value regularly increase or decrease， and the position of secondary peak regularly move forward or backward.