Abstract: In order to reveal the cause of the dynamics stability lost phenomenon in the steep inclined ultra thick seam of Dahonggou Mine, based on the theoretical analysis and rock mechanics experiments of the conducted on the mining and excavation distribution features, the paper analyzed causes of the deep coal and rock d ynamics instability in the steep inclined seam.The mining depth, the roof and floor holding function, roof and floor broken, coal and rock physical mechanics properties, stress concentration and other factors were included.The results showed that the prying role of the floor would be the dynamic sources of the seam instability and the s eam had the condition to occur the dynamic.The B3+6 seam mining face cased the pilot pressure ovelying between the driving face and the coal mining face.The stress concentration was serious.The blasting pressure releasing and water injection weakening should be conductal in the seam and the roof and floor to release the elastic e nergy acsumulated in the coal and rock and to optimize the mining and excavation layout.