In order to solve the problem of poor precision for measuring C/ C composite elastic constant by traditional test method, the present paper provided a new method for obtaining the main elastic constants from modal test. According to the modal test data and the relationship between vibration frequencies and elastic constant, the scope of the elastic constants were estimated preliminarily. Then the quadratic response surface approximate model about elastic constants and vibration frequencies was established to optimize the elastic constants. The isotropic and orthotropic thin plates were used to verify this method. The results show that the maximum error is 16. 67%, the minimum is only 0. 5%, so the method is accurate and effective. At last, the elastic constant of needled C/ C composite is obtained through the present method. The simulation and experimental results proved that the method is nondestructive,efficient and accurate.