Abstract: The comparative numerical experiments with real and climatic sea surface temperature (SST) for three cases are performed, by using the global spectral model. These cases are on July 10, July 19, 1992 and July 1, 1997, respectively. The effects of real SSTs on the monthly dynamic extended forecast are investigated by the different circulation changes in the general circulation model generated with real and climatic SSTs. The results of numerical experiments show that the effect of real SST on predictions less than 10 days is not obvious, but the impact on monthly forecasts is obvious. The forcing effect of real SST on the monthly forecasts is complex and may be related to the initial conditions of numerical integration, the various physical processes included in the forecast model, and the strength of SST anomalies.