Using the NCEP/NCAR reanalysis data during 1980-2004,some forcing factors,such as latent heat release from condensation and underlaying surface thermal differecence,are defined and their impacts on the south-to-north cross-equatorial flow are studied with diagnosis analysis and numerical simulation.Main conclusions are shown as follows:the cold Australia land always corresponds to strong Australian High or frequent cold air activities,which is favorable for the formation of strong cross-equatorial flow;the cold underlaying surface to the south of the channel is often accompanied by weak Walker circulation,which is also helpful for the establishment of strong cross-equatorial flow;the warm Indo-China peninsula and cold South China Sea,as well as the strong heat source near the Philippines,all can be associated with strong convergence zone in north hemisphere,which is advantageous for the formation of strong cross-equatorial flow.The features of forcing factors and circulation factors corresponding to weak south-to-north cross-equatorial flow are opposite to the above ones.