Cracks that occurred in the top slab of a cantilever construction bridge were investigated through field test, which revealed that the cracks were caused by excessive prestress layout and too small curvature radius of tendons. The excessive prestress layout can reduce the forced area of cross-section. Too small curvature radius of tendons may cause the concrete in the flat curve of tendons to withstand great radial component of forces. And, excessive prestressed ducts will in a certain extent affect the quality of concrete between the ducts. In addition, a finite element analysis was performed to evaluate the effects of the tendons. Based on the results, a construction method that prevents the cracks is proposed.