Abstract: This paper analyzes the abnormal conditions of large-scale circulations, convection, and water vapor transportation and their effects on Yunnan precipitation at various MJO (Madden and Julian Oscillation) phases in May during 1979-2008. During this period, Yunnan precipitation showed obverse differences at eight MJO phases that were divided by the central location of MJO from west to east. Positive precipitation anomalies occurred at phases 4-6, where the main MJO convective body was situated along the equator from the central Indian Ocean to the western Pacific Ocean. Negative precipitation anomalies appeared at phases 7-8, when the main MJO convective body appeared east of the equator at the central Pacific Ocean, and at phases 1-3, when it was situated over the central and western Indian Ocean. The positive anomalies at phase 6 and the negative anomalies at phase 2 are the most significant. The main convective body moved northward and eastward from the tropical Indian Ocean. At phases 1-3, the southwestward water vapor transport had not yet formed in the Bay of Bengal (BOB); therefore, the precipitation in Yunnan at various divergent regions decreased. At phase 4, the main convective body moved near 90°E, and a partial convective cloud system began a northward transfer to the BOB. As a result, cyclonic circulation formed and transported water vapor to Yunnan; therefore, its precipitation showed positive anomalies. At phase 5, the main convective body moved to the South China Sea (SCS), from which a partial convective cloud system began a northward transfer with northward cyclonic circulation. At phase 6, the main convective body of the equatorial MJO had moved eastward from the BOB. However, the two cyclonic circulations in the BOB and SCS continued northward, and the southwestern wind along the eastern cyclone of the BOB and the northeastern wind along the western cyclone of the SCS met and joined in Yunnan. Because they were controlled by a strong water vapor convergent region, and precipitation in this region shows the highest increase. At phases 7-8, the main convective subject was weakened and moved eastward to the SCS and the western Pacific Ocean. The wind in the BOB shifted to the north and no longer transported water vapor to Yunnan, where it was controlled by divergent water vapor to decrease precipitation. Therefore, northward convection propagation along paths in the BOB and SCS during the eastward migration process of MJO created strong water vapor transportation. Their advantageous coordination in the two sea regions is the main reason for Yunnan precipitation in May.