The oasis evolution and human activity strongly related to hydrological conditions in Tarim Basin,Xinjiang, China, in historic time. The Lop Nur(Loulan) region, southern Xinjiang, was the terminal lake of the Tarim River and the famous ancient Loulan kingdom located on the west bank of the Lake Lop Nur. However, the area has been experienced dramatic changes from oasis to wasteland. In this paper, we conducted systematical radiocarbon dating(AMS ~(14)C)with 30 specimens of subfossil Populus euphratica lying on ancient riverbeds,which may be driftwoods transported by floods or be dead trees grew on both sides of the river, 10 specimens of Tamarix chinensis and reed in Yardang area between ancient rivers,and 5 specimens sampled from three archaeological sites (two settlements and an artificial canal). The statistical characteristics of ~(14)C ages were also analyzed. Finally by compared with historical documents and reported palaeoclimatic records,the relation between oasis environment and human activity was discussed. Results show good consistency between natural plant remains and archaeological sites, corresponding with the Yuan-Ming Dynasty ( 1271 ~1644 A.D.). It was found that the difference on age distribution of two aforementioned sources of subfossil P. euphratica is obvious. The presence of three age peaks of P. euphratica,corresponding with our driftwood samples, implies that the Loulan area experienced at least three flood events and re-experienced oasis environment from 1260 cal. A.D. to 1450 cal. A.D., which is the climate transition stage from the Medieval Warm Period to the Little Ice Age. Specifically, results show that reed and tamarisk samples appear to be dated to the end of flood events,indicating a decrease in groundwater level during intermittent periods. Therefore,it is concluded that several pulse-like flood event-featured wet-dry alternative cycles constitute the wet period. In the meantime, human re-settled in the Loulan area and built canals to irrigate farmlands. Buried reed layers were found on the artificial bank because the people who lived in the Loulan area at this time dredged the canal and placed sediments directly onto growing reeds in the banks of the canal during the flooding periods. The presence of reddish sintered-clay breccia in the artificial bank can also be interpreted as evidence of human activity at this time, which may be related to people maintaining the canal after flooding events. By compared with high-resolution palaeoclimatic records( ice core, tree ring, and stalagmite) from Kunlun Mountains, Qilian Mountains, and Tianshan Mountains, around Tarim Basin, it was believed that human resettlement in the area during the Yuan-Ming Dynasties was primarily dominated by an increase in runoff resulted from mountain precipitation. Furthermore, Lop Nur region is the core arid area of Eurasia, the enlargement of Loulan oasis strongly suggests the wet climate and a prosperous oasis environment in the whole central Asia,that may be a major environmental factor for the three times of Mongol westward conquest in 13~(th) century.