Abstract: Seasonal evolution of maximum solar radiation (MSR) from monthly observations of the solar radiometer (model: EKO-MS6020) at 0°—25° angles in Xinzhuang of Shanghai is analyzed and the MSR biases of the European Center for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis product during 2007—2021 are evaluated. In addition, linear relationships of MSR with various meteorological variables as well as favorable synoptic circulation backgrounds are revealed. The seasonal fluctuation range of MSR in Xinzhuang of Shanghai is 800—1300 W/m², and the maximum and minimum values appear in May and December, respectively. Results show that monthly MSR is close to the solar constant with an interannual variance of 200 W/m². When the inclination angle is within 5°—20°, the received monthly radiation is about 50—250 W/m² more than that with the angle of 0°. The optimal inclination angle is 20°, corresponding to the best income of MSR. The ERA5 atmospheric reanalysis shows a significant underestimation of MSR with an average annual underestimation of nearly 200 W/m². Although their correlation coefficient is 0.88, it is mainly due to the seasonal cycle. Significant biases of ERA5 appear both in spatial and temporal domains, and their interannual anomalies are not significantly correlated with observations. Based on temporal evolution of synoptic circulation background with a time difference of fewer than 3 d, we find that the synoptic pattern with stronger northerly winds, less cloud cover, and high temperature is generally favorable for MSR, although the circulation structure is different in spring, summer, autumn, and winter.