Abstract: In order to solve the problems of low accuracy and abnormal error in numerical simulation method for stability analysis of large-scale rock mass engineering， a more than 100 million degree-of-freedom(DOF)numerical simulation technique was developed. Taking the surface rock movement engineering of open-pit and underground combined mining at Dahongshan iron mine as an example， this technique is elaborated in detail. Combining the simulated results with the fissure of ground surface range， a method of the fissure of ground surface range inversed by fuzzy surface subsidence value is proposed. Combined with the more than 100 million DOF numerical simulation technique， this method accurately estimates the development of the fissure of ground surface range. The absolute error of position is 1.59~4.71m with a span error rate of 1.141%， verifing the reliability of the method and improving accuracy compared to the commonly numerical simulation method of millions DOF. At the same time， the fissure of ground surface range is estimated after 5 years. With the mining of underground 300~720m level ore body five years later， although the waste rock filling of collapse pit can inhibit the deformation of surrounding rock， the surface movement and cracking range will still affect the open-pit， and it is estimated that the crack area will affect the range above 990m steps at least in the open-pit.

Key words: more than 100 million DOF; numerical simulation; refined modeling; open-pit and underground combined mining; surface subsidence