A pre-distortion fringe deflection approach based on reverse ray tracing was proposed to address the blurred edges and low contrast of the fringe pattern captured by the camera while measuring a convex sphere with a small radius of curvature (ROC) using phase measuring deflectometry. In this approach, the pre-distortion fringe pattern was obtained by employing the reverse ray tracing model based on the nominal ROC of the spherical element to be tested. N-step phase shift algorithm and iterative optimization algorithm were applied to obtain the coordinates and height of each point on the element surface and subsequently combine with the differential geometry approach to compute the mean ROC of each point of the element. For a convex surface with an ROC value of 8 mm, the measurement accuracy in numerical simulations was approximately 11 μm. Finally, the mean ROC obtained for the points on the convex sphere with a ROC value of 8.26 mm was 8.28 mm, and the diameter of the measurement area increased from the original 4 mm to 5 mm. The findings reveal that compared with the traditional deflection approach, this approach can not only overcome the issue of blurred edge and low contrast of the fringe pattern when measuring convex spheres with small ROC but also improves the measurement accuracy and effective measurement area.