In this paper, a method for measuring the elastic modulus of cross-ply composite plates based on Lamb waves is presented. Cross-ply composite plates are equivalent to Single-layer orthotropic plate with 9 independent elastic constants. The sensitivity of group velocity of S0 Lamb waves to elastic constants is studied in this paper. It is found that the group velocity of S0 Lamb waves is only related to the tensile elastic modulus and the in-plane Poisson’s ratio of the orthotropic plate at the low-dispersive frequency-thickness products range. By analyzing the change ranges of equivalent engineering elastic constants for cross-ply composite plates, it is discovered that the change of in-plane Poisson’s ratio of cross-ply composite plates is small enough to ignore its effect on the group velocity of S0 Lamb waves. Therefore, the group velocity of S0 Lamb waves can be used to estimate the tensile elastic modulus of the cross-ply composite plates, and the mapping relationship between the group velocity of S0 Lamb waves and the tensile elastic modulus can be established. The method has been verified by numerical simulation and experiment and the error between the tensile modulus and the actual value of the composite plate obtained by this method isless than 10%, which provides convenience for the measurement of relevant structural parameters in aerospace and other industrial fields.