This study proposes a detection method for the 248 nm depolarizer independent of laser intensity fluctuation. We selecte an excimer KrF laser with a stable output wavelength of 248.372 nm as the light source. The adverse effects caused by the light intensity fluctuation are overcome through pre-polarizing and polarization splitting design. The error analysis shows that the proposed method is suitable for performance detection of the depolarizer. The absolute system error is less than 0.1226%, and the random error is ~0.2000%. The verification experiments show that the measured mean DOP is highly consistent with the theoretical value. It also shows that the measurement system error is only 0.0423%, confirming the validity of the proposed method. The stable test shows the repeatability level (3σ=0.2316%) is consistent with the analysis expectation (~0.2000%), and the reproducibility level (3σ=0.3090%) is only slightly worse than the repeatability level, indicating the stability of the device. Therefore, the proposed method provides a stable and effective tool for offline performance detection of the depolarizer used in the 248 nm lithography tool illumination system. Furthermore, the method is essential and should be further developed to meet the requirements of shorter wavelength lithography systems.