Abstract: Ionospheric delay represents one of the major error sources in GNSS positioning. To develop a more accurate ionospheric model, this study establishes a global ionospheric model by integrating multi-GNSS observations. Data from Multi-GNSS Experiment (MGEX) stations under various geomagnetic conditions were utilized to systematically evaluate the performance of spherical harmonic-based ionospheric modeling with different satellite system combinations. Experimental results indicate that during periods of low solar activity, the integration of GPS, BeiDou-2 Navigation Satellite System (BDS-2), and BeiDou-3 Navigation Satellite System (BDS-3) data improves modeling accuracy compared to using GPS/BDS-2 or GPS/BDS-3. Specifically, the mean absolute deviation (MAD) decreases by 0.08 TECU and 0.04 TECU, the standard deviation (STD) decreases by 0.22 TECU and 0.19 TECU, and the root mean square error (RMSE) decreases by 0.19 TECU and 0.16 TECU. During high solar activity periods, even more substantial enhancements are observed: the MAD decreases by 1.61 TECU and 0.10 TECU, the STD decreases by 0.39 TECU and 0.19 TECU, and the RMSE improves by 0.99 TECU and 0.94 TECU, respectively. Furthermore, the combined GPS/GLONASS/Galileo/BDS/Quasi-Zenith Satellite System (QZSS) system shows remarkable performance advantages over the GPS/GLONASS/BDS combination. During geomagnetically quiet conditions, the MAD, STD, and RMSE decrease by 0.39 TECU, 1.12 TECU, and 0.97 TECU, respectively. Under geomagnetic storm conditions, these three evaluation metrics decrease by 1.97 TECU, 1.58 TECU, and 2.42 TECU, respectively. In conclusion, multi-GNSS data effectively enhances ionospheric modeling accuracy and provides more reliable data support for high-precision navigation positioning and space environment monitoring.