The MODIS precipitable water vapor (PWV) product has been widely used in hydrometeorological and hydro-ecological research because of its kilometer-scale spatial resolution, daily temporal resolution, and global coverage. However, its regional applicability, error characteristics, and correction methods remain insufficiently evaluated in arid regions because radiosonde (RS) stations are sparsely distributed and their routine observations at fixed times rarely coincide with MODIS overpasses. Based on RS observations and two Global Navigation Satellite System (GNSS) meteorological stations over the arid Hexi Corridor, we established a two-stage method in which fixed-time RS observations were first used to identify the most suitable GNSS-PWV retrieval scheme and correct the resulting estimates, after which the corrected GNSS-PWV series was used to evaluate MODIS-PWV at satellite overpass times. Results show that the GNSS-PWV retrievals were sensitive to weighted mean temperature (T<sub>m</sub>) estimation, and those obtained using integrated T<sub>m</sub> agreed better with RS-PWV. After RS-based linear correction, the corrected GNSS-PWV achieved R = 0.97 and RMSE = 2.30 mm. Relative to the corrected GNSS-PWV, MODIS-PWV showed a systematic overestimation, with mean biases of 2.21 mm at the reference station GSIA and 2.88 mm at the transfer-test station GSCM, and larger biases during high-PWV periods in summer. A univariate linear correction can effectively reduce the bias, with the RMSE decreasing from 3.47 to 2.05 mm at GSIA and from 5.22 to 3.58 mm at GSCM. This two-stage RS–GNSS–MODIS method provides a methodological reference for evaluating satellite-derived water vapor products and supporting their regional application in arid regions.