Prepregs are prone to wrinkle defects during complex double-curved surface forming due to excessive in-plane shear strain, significantly compromising the mechanical performance of components. This study establishes a non-orthogonal constitutive model based on the continuum mechanics theory, employing a non-orthogonal coordinate system to track yarn orientation changes and characterize the in-plane shear and deformation behaviors of prepregs during double-curved surface forming. The mechanism of wrinkle formation is analyzed, and a wrinkle quantification method based on normal direction variation rate is proposed. Combined with boundary constraint conditions via blank holder force, the inhibitory effects on wrinkle defects are explored. Results demonstrate that as the blank holder force increases from 25 to 100 N, the wrinkle area decreases from 1377.5 mm2 to 1125.7 mm2, while the maximum shear angle rises from 35.5° to 36.9°, confirming the mechanism by which blank holder force suppresses wrinkles through enhanced shear deformation resistance and optimized stress distribution.