Abstract: The exploitation of underground mines generally has an effect on the earth's surface in some degree.To ensure safety and provide basis of calculation，probability integral method and flac-3d numerical simulation are applied to predict the area of surface subsidence according to the actual engineering condition that an iron ore will expand mining boundary with the cementing filling method.Based on nine prospecting lines，the surface deformation numbers is taken as the reference standard to obtain the maximum surface deformation area.A simulation model of mine is established via Flac-3d software to reflect the inner properties around the mining information of three-dimensional geographic，geologic information and formations.This method simulates the surface deformation in the process of dynamic excavation and eventually determines the surface deformation contour map.Expanding mining has little influence on the normal production of enterprises and residential life when the settlement value near the mining center is 18 mm，settlement value is under cm level around enterprises and residential buildings is out of settlement area at 2 mm.The surface monitoring data further verifies the accuracy of calculation above.Calculation of surface influence area for underground mining expanding can guide production and ensure safety in industry，and own a profound significance.

Key words: Underground mining, Surface subsidence, Probability integral method, Numerical simulation, Surface monitoring