Based on the quasi-steady-state model and momentum theory, the calculation of the induced angle and the optimal geometric angle of attack of the flapping-wing micro air vehicle was discussed. Aiming at the non-convergence problem of the Gauss-Newton method in solving the induced angle equation, the Levenberg-Marquardt algorithm was used to guarantee the convergence of the calculation process, and the induced angle of the experimental prototype is obtained as 15°. The practical lift of the experimental prototype is 40g, while after ignoring the induced angle, the theoretical lift of the quasi-steady-state model is 46g, and the theoretical lift considering the induced angle is 37g. It is verified that calculating the induced angle can improve the accuracy of the quasi-steady-state model for predicting lift. According to the actual linear twist of the wing, the least square method is used to calculate the actual best geometric angle of attack at about 55°, which can effectively guide the design of the wing, thereby improving the maximum lift and flight efficiency of the flapping-wing micro air vehicles.