Abstract: The in-plane shear deformation behavior of unidirectional thermoset prepreg has significant effect on the forming quality and mechanical properties of composite parts after hot diaphragm forming. In this paper, the in-plane shear and stress relaxation behavior of unidirectional thermoset prepreg at different forming temperatures and loading rates were investigated. The test results show that the unidirectional thermoset prepreg exhibits a nonlinear in-plane shear deformation behavior strongly related to temperature and loading rate, and the sensitivity of shear deformation to loading rate decreases with the increasing of temperature. The relaxation rate of unidirectional thermoset prepreg accelerates with the increasing of temperature, and the sooner it is in a stable state. Based on the in-plane shear stress relaxation behavior of unidirectional thermoset prepreg, a generalized Maxwell viscoelastic constitutive model was constructed, which could accurately track fiber orientation change. The viscoelastic constitutive model was implemented in the user material subroutine VUMAT in Abaqus. The off-axis tensile stress relaxations were simulated. It is in good agreement with the experimental results, indicating the effectiveness of the viscoelastic constitutive model.