To further enhance the service performance of bridges and prolong structural service life, it is of critical importance to effectively detect the prestress of structural steel strands in bridges. Conventional detection methods have limitations in practical engineering applications, showing substantial variabilities in test results. The X-ray diffraction (XRD) method, based on the microscopic scale analysis of metallic materials, derives macroscopic stress distribution through lattice spacing variations, demonstrating theoretical clarity and testing stability. This study pioneered the application of XRD in the field of effective bridge prestress detection. Comparative laboratory tension tests were conducted to investigate surface treatment process optimization and correct effective stress. The results demonstrate that the XRD method exhibits remarkable stability in prestress detection, with advantages including diverse application scenarios, low result variabilities, and high detection accuracy. Optimal surface treatment is achieved through grinding with hard abrasive wheels in combination with two-minute electrolysis in a 15 V lead-acid solution. Both linear and exponential correction formulas for effective stress grading are proposed. The research outcomes provide references for implementing XRD technology in bridge prestress detection.