Abstract: From the field investigation,the authors discovered the massive rock mass with cleavages in northwestern Yunnan.The spatial distribution and occurrence of cleaved rock mass are controlled by the regional activity faults.From Yanjing in Tibet to Lijiang in Yunnan,there are three densely cleaved rock mass zones which are arrayed in right en-echelon formation and are consistent with the orientation of the Yunnan-Tibet railway in-northwestern Yunnan.In northwestern Yunana,the endogenetic force is closely coupled with the exogenetic force.The massive cleaved rock mass has special and complex property.Based on the fieldwork and analysis,this paper discusses the distribution and property of the massive cleaved rock mass in detail.The effect of the cleaved rock mass to the railway projects is analyzed to prevent geologic hazard,gushing water in tunnelling,distortion burst in the surrounding rocks.The discovery and analysis are useful to the railway projects in northwestern Yunnan.