To verify the reasonability of calculating the flexible rocket’s aerodynamic forces based on the quasi steady theory and the linear theory when both the deformation and the attack angle are small enough, the computational fluid dynamics (CFD) method is used to calculate the rocket’s lift and drag coefficients. Using this method and the empirical formula, the rocket’s aerodynamic coefficient derivatives distributions are obtained. The transfer matrix method of the linear multibody system dynamics (MSTMM) is applied to get the rocket’s frequencies and mode shapes. Under the assumption of quasi-steady theory and linear theory, using the obtained aerodynamic coefficient derivatives distribution, the aerodynamic forces of two bended rochets’ the deformations of which are based on the first order mode shape are calculated. To judge the reasonability of the linear theory, the CFD method is also used to simulate the two models. Compared with the CFD method, the proposed method has high precision under the practical flight conditions. In addition, when the bending degree is much higher than the possible deformation, the precision of the rocket’s nose is still fine but very low in the tails position for the proposed method.