The mechanical behavior of composite material is greatly affected by the copulate layer between neighboring phases. In present paper three parameters are introduced to describe copulate state between phases. By means of equivalence principle and method in multiphase solid theory, formula containing the copulate state parameters are developed to predict the FRP elastic constants. In the paper the auter proposes that two-or three-test-design methods combining with the formula will approach satisfactory result in predicting FRP elastic constants.