Abstract: In order to study the mechanism of cemented backfill acting on the second-step ore body，and taking 782，792，and 802 three one-step stopes of the Xincheng Gold Mine as research object，the FLAC3D simulation software was applied to make simulation on the mechanism of cemented backfill from the third segment to the fifth segment for the excavation and backfilling.The project result indicates:The stress of second-step ore body is 5~6 times as big as that of cemented backfill during excavation and backfilling from the third segment to the fifth segment of first-step stope.Tensile failure was produced in plastic zone of contact surface of cemented backfill and second-step ore body in the 792 stope，and shear failure was produced in plastic zone of contact surface of cemented backfill and second-step ore body in the 782 and 802 stopes，and their depth remained at around 1m.After one-step mining was excavated at layer 11，the subsidence of roof increased quickly，so roof should be supported to ensure mining safety.

Key words: Cemented backfill, Two-step ore body, Mechanism, Numerical simulation