Southeastern Yunnan has 4,996 species and varieties of 1,357 genera and 186 Families of native seed plants recorded. Floristic attributes and biogeographical affinities of the flora were studied by analyzing its floristic composition and geographical elements. Tropical genera comprise a majority (68.83%) of the flora and those of tropical Asian distribution contribute to 27.34% of the total genera. The flora of southeastern Yunnan is similar in composition to the floras of southern Yunnan, southwestern Guangxi and Vietnam. They have similarities of more than 89% at the family level and more than 76% at the generic level. The flora of southeastern Yunnan, with the compared floras together, belongs to the same floristic unit and is suggested to be part of Indo-Malaysian flora at northern margin of tropical Asia. However, the taxa of strictly tropical distribution are still underrepresented in the flora of southeastern Yunnan compared to Indo-Malaysian flora, and the families of mainly subtropical to temperate distribution, such as Magnoliaceae, Theaceae, Cornaceae, Styracaceae, Symplocaceae, Aquifoliaceae and Caprifoliaceae, are well represented in the flora. Some characteristic families of temperate East Asia, Such as Diapensiaceae, Dipentodontaceae, Eupteleaceae, Grossulariaceae and Toricelliaceae are also present in the flora of southeastern Yunnan. These suggest that the flora of southeastern Yunnan is related to Eastern Asian flora more than other compared floras. It is also revealed that the flora of southeastern Yunnan is biogeographically closer to the flora of southwestern Guangxi than that of southern Yunnan.