This study presents a technique for measuring the centering error of a lens. The technique uses power analysis and is highly accurate. The module is designed by a prism that is patterned using a subwavelength grating combined with a reflection-centering system by using the -1st-order rays at 5 mW and 650 nm. The optical axis error is tested and analyzed by the powermeter by using Snell's law and diffractive characteristics. The resolution of the optical-centering error in the system is amplified by a factor of 4 compared with the centering error measuring system based on autocollimation. The technique proposed in this study improves the sensitivity of the instrument and reduces the requirements of the sensor at the end side based on the direction or energy variation of the light.