Abstract: Utilizing correlation analysis, diagnostic analysis and numerical simulation experiment, influence of sea surface temperature abnormality of two tropical oceans on rainfall during the season when dryness turns to wetness in early summer in Yunnan is studied. The results show that the preceding phase sea surface temperature anomaly has an obvious effect on the rainfall and the distinct correlation area between SST and rainfall is corresponding to the branches ascending and sinking of Walker circulation. The longer the time scale is, the less predictable information the oceans provide; the shorter time scale, the less predictable information. Diagnostic analysis and numerical simulation results also show that monsoon depression in the Bay of Bengal, cross-equatorial flow in Somali in early summer and southwest monsoon in India weaken during warm phase of equatorial central and eastern Pacific. Otherwise, they strengthen. Sea surface temperature variations affect monsoon activity through ocean-atmosphere interaction, then affect the rainfall.