Abstract: In order to make a clear on the enrichment causes of the major seam in Xishanyao Formation of Santanghu Coalfield,based on the drilling,logging,paleontology and information of Santnghu Coalfield,with the sedimentary facies,sedimentary rock superimposed mode,lithofacies and other variations,the Xishan Formation was divided to a four grade bed succession,a three system tract of high level,lake transgression and low level and a stratigraphy framework of Xishanyao Formation in Santanghu Coalfield was established.The study result showed that the material sources of Santanghu Basin were mainly came from Dahaputike Mountain in the Northwest,Suhaitu Mountain in the North part and Keziletage Mountain in the Northeast and The deposition center of the basin was located in the center of Hanshui Spring and the sag zone of Tiaohu.In this period,the coal formation environment mainly were the moist forest-swamp facies and shallow water forest swamp facies and the coal forming plants were composed of the woody plants,mainly including the cyatheaceae,cycadaceae and pinaceae and the herbaceous plants,including little lygodiaceae,osmundaceae and others.The seams were mainly distributed in the maximum lake flooding surface nearby the lake transgression system area,the delta bay and lake bay of the braided river and the delta plain area of the downstream braided river would be the most favorable to the coal accumulated deposition environment.