<p>ERA5, European Centre for Medium-Range Weather Forecasts Reanalysis Fifth Generation is a significant dataset in climate reanalysis. This study evaluates the global simulation performance of ERA5 data (2020–2023) using data from 9,957 in-situ observation stations, which is crucial for a deeper understanding of extreme climate change and its applications in climate monitoring, GNSS-based atmospheric studies, and weather prediction. The study analyzes variations in temperature, pressure, specific humidity, and relative humidity across daily, monthly, and seasonal scales, with particular attention to the influences of altitude and latitude. Statistical methods such as root mean square error (RMSE), correlation coefficient (CC), bias, and slope were used for comprehensive performance evaluation. The results show that: (1) The RMSE of temperature ranges from 2.5°C to 3.5°C, with a CC between 94% and 98%, and a bias between -2°C and 0.2°C. The RMSE of pressure ranges from 3.9 to 4.3 hPa, with a CC between 99.72% and 99.78%, and a bias between -0.5 hPa and 0.1 hPa. The RMSE of specific humidity ranges from 1.5 to 3.5 g/kg, with a CC between 93% and 99%, and a bias between 0 and 3 g/kg. The RMSE of relative humidity ranges from 10% to 15%, with a CC between 70% and 90%, and a bias between -4% and 4%. (2) ERA5 meteorological variable errors exhibit clear seasonal and latitude effects, with temperature bias ranging from 0.161 °C in summer to 1.136 °C in winter (RMSE 1.072–1.862 °C), pressure bias reaching 15 hPa in some high-altitude regions, and specific humidity errors peaking at 8 g/kg near the equator and −2 g/kg in polar regions. These findings deepen the understanding of the performance of ERA5 data and may serve as a useful reference for future evaluations of ERA6 when relevant data become available.</p>