The R-angle area of C-beam is prone to fold defects in the process of autoclave forming, which seriously affects the forming quality and service performance of the component. In order to accurately evaluate the degree of fold defects and provide ideas to reduce the generation of folds, C-beam samples of two different brands of prepregs at home and abroad under different layers were manufactured by using autoclaves forming process. The deformation behavior of R-angle prepregs in the forming process was analyzed and the mechanism of fold defects formation was explored. An evaluation method including thickness distribution, edge slip angle and R-angle quantitative analysis of partial folds is proposed, and the samples are evaluated and compared by using this method. The results show that the interlayer slip behavior of prepreg is the key mechanism of fold formation, and excessive slip resistance leads to fold defects. Compared with foreign prepregs, the tested domestic prepregs have greater overall viscosity, and C-beams manufactured are more prone to fold defects. The three evaluation indexes are consistent, and the degree of fold defect at R-angle of C-beam can be measured comprehensively from multiple angles.