Chengmenshan open pit has been constructed to be the biggest copper pit in China. The slope designed with empirical rock mass rating and engineering analogy needs reinvestigation and evaluation for its overall stability. With help of SURPAC, the topographic contour of the mine was converted into surface model. Then the geological information of the mine was embedded into the numerical solid model by NURBS treatment of the core loggings. Through overall stability analysis with MIDAS, it was found that there was no obvious failure in the pit with the current design. But the major principal stress at the north especially the north-east corner of the pit tended to be high, so the width of the bench below the high stress region is suggested to be widened for possible collapse in this region. And field observation should be carried out in the region. Once evident slip or collapse happens, anchor and netting structures should be used. At the same time, high shear stress zones were revealed at the south-east of the outside limit of the pit. More planting is suggested to avoid soil creep or even landslides in heavy rain.