Abstract: Through theoretical derived and finite element simulation, the vibration characteristic of delaminated plain woven fabric glass fiber/epoxy composite laminate with four sides clamped and one side clamped under hygrothermal environment were studied. This paper studies the relationship between delamination angle, delamination location and vibration characteristic. Based on the first order shearing deformation theory of Mindlin and Hamilton principle, the formula of hygrothermal constitutive equation and the natural frequency of laminated plates was derived, considering the hygrothermal stress, mass effect and the equivalence between moisture and temperature. Using user material subroutine (UMAT) interface with the finite element software ABAQUS, the corresponding interface program was compiled and a series of delaminated model under hygrothermal environment were established. It is found that the simulation results are in good agreement with literature experimental value. Results show that delamination makes vibration frequency decrease intensify under hygrothermal environment. Increasing delamination angle can cause the decline of frequency value, and the larger delamination angle, the larger decline frequency. The influence of moisture is more serious than that of temperature on vibration characteristic. The hygrothermal effect can cause the order of the base frequency decline and intensify the local resonance phenomenon. Surface delamination and middle delamination can cause modal change, but there is little difference.