Abstract: This paper describes the dynamic hydroelastic modulus E’ measured in the experiment and presents the formula of the dynamic hydroelastic modulus. Experimental results show that there are three stages for the dynamic hydroelastic modulus with vibratory frequencies: linear, transitive and horizontal. It is found that the added mass theory is rather limited. When EI>>E’I’, the formula of the dynamic hydroelastic theory may be simplified into the formula of the added mass theory, and only when the added mass theory is right.