Abstract: The working face 11-101 of Tuanbai coal mine in Huozhou, Shanxi Province is exploited above aquifer. In order to prevent floor water inrush accidents during mining, research combination of elasto-plastic theory, on-site measurement and numerical simulation was carried out to analyze the development rule of failure depth of the floor, then the reasonable length of the mining face under this condition was determined. The result shows that with the advancement of working face, the bottom plate of lagging working face is stretched and destroyed at about 6 m behind the lagging working face, and the bottom plate has a wedge shape. The length of the working face is in the range of 60 m to 120 m. The damage depth increases linearly with the length of the working face. When the length of the working face increases every 10 m, the damage depth increases by 1 m, and the reasonable mining length of the first mining face is 80 m. Theoretical calculations and numerical simulations show that the floor damage depth of the 80 m working face is 7-8 m. The actual measured failure depth of the floor is 7.5 m. Therefore, when the reasonable mining length of the working face is 80 m, the production safety of the working face 11-101 can be ensured.