This paper introduces a method for measuring the elastic modulus of cross-ply composite plates based on Lamb waves. Cross-ply composite plates are equivalent to a single-layer orthotropic plate with 9 independent elastic constants. The sensitivity of the group velocity of S₀ Lamb waves to elastic constants is studied. It is observed 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 in the 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 a 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 is less than 10%. This provides convenience for the measurement of relevant structural parameters in aerospace and other industrial fields.