Abstract: By means of the introduction of the classic laminate theory with considering hygrothermal effect into the finite element method, the mechanical properties of composite in hygrothermal environment were accurately forecasted and the damage evolution was tracked through careful consideration of equivalent lamina parameters reduction and unloading after initial damage and estimation of final damage. The tensile and compressive strengths of three different lay-up composite laminates in hygrothermal environment were studied. There is good agreementbetween experiment and simulation results.