Abstract: Accurately understanding of the open-pit mining bench security and the stability of mined-out area is an important prerequisite for work safety in mines. In order to analyze the relationship between the safe thickness of open-pit mining platform and the span of hidden goaf，a series of experiment of rock mechanics was conducted to obtain the basic physic mechanical parameters of the ore rock samples. Combined with Mohr-coulomb criterion，based on the condition of different span of goaf area，specified length of 30 meters of goaf area，with the progress of 10 meters every time to excavate up the worked out section，the three-dimensional simulation model was built up by FLAC3D，and a linear fitting formula as the result was put forward. The result of research has the reference significance for mines with similar rock properties and production condition.