Abstract: The grey theory is applied to the ionospheric foF2 short-term forecast. According to the theory that the optimum grey forecasting length of the sample sequence can be determined, the forecasting model, which based on the residual error correction, is constructed, and the observation data of multiple ionospheric observation stations in China are adopted for test. The research shows that the results of the average grey range information entropy can reflect the cyclical effects of solar rotation. In addition, the forecasting method is more precise in high latitudes than in lower latitudes, and its error is relatively larger during seasons with intensive solar activity. Besides, the average relative residual of the effects forecasted 1 day in advance is less than 1 MHz and its average precision is more than 90%. This method provides a new way of thinking for the short-term forecast of the ionospheric foF2 in the future.