Abstract: In order to fully understand the deformation behavior of prepreg in the process of hot diaphragm forming, the in-plane deformation mechanism of unidirectional carbon fiber/epoxy thermoset prepreg at high temperature was studied using the bias extension method. The effects of testing temperature, tensile rate, preheating time and stacking sequence on the in-plane deformation were investigated. Using digital image correlation technology, the specimen deformation and fiber rotation were captured during testing process. The results show that increasing temperature or decreasing tensile rate can promote the deformation of unidirectional carbon fiber/epoxy thermoset prepreg. Stacking sequence has a great influence on the deformation behavior of the unidirectional carbon fiber/epoxy thermoset prepreg stacks. The fiber rotation of 45/–45/90_S layup is easier than that of 45/90/–45_S layup, and the former has lower deformation resistance. The Pin-joined net (PJN) theory was used to predict the fiber rotation angle during the deformation process of the unidirectional carbon fiber/epoxy thermoset prepreg stacks, and the prediction value was compared with the experiment value. The results show that the prediction values deviate greatly from the experiment values, indicating that the PJN theory is not suitable for the prediction of the fiber rotation angle during the deformation of unidirectional carbon fiber/epoxy thermoset prepreg. Being preheated at 80℃ can increase the deformation resistance of the prepreg.