Abstract: Chang’anying Village of Chengbu County is a typical southwest mandarin dialect islanxdl.'Through the methods of feld investigation and historical literature ,this paper inv estigates the source ofsouthwest mandarin in Chang'anying village and the causes of dialect island.The formation of the south-west mandarin has the closest relationship with the historical immigrants. 'The officers and soldiers fromall over the country lrought the languages from all over the country. They const antly contacted，influ-enced and gradually menged with each other , and gradually formed a umique umique language of the mixedaccent in a new settlement area.Among them , the Zhengan dialect , which has the prototype of the south-west mandarin , was the most influential，thus gradually foming the soutlwest mandarin. 'The fomation ofthe dotted dialect island is closely related to the national policy of the Qing Dynasty and the natural envi-ronment : in terms of national policy , the govemment does not allow military and civilian exchange withlocal ethmic groups and prohibits the spread of ethnic group languages;in terms of physical geography,the climatic conditions of Chang'anying Village are relatively harsh. There is only one ancient road lead-ing from Chenglw County to Chang anying Village，which was built for transporting troops and grain afterthe establishment of Chang'anying Village.Because of this，the officers anxd soldiers who come and golive in a small area. 'They form their own communities and have little contact with the outside world. Local people generally do not intervene in the immigrant community. A closed immigrant society has beenformed，and the language formed in the region is like an island surroumnded by the sea.

Key words: southwest mandarin, dialect island;immigrant , physical geography