Abstract: During the last six decades of dewatering, the cone of depression of groundwater level was formed and the saturated zone was transformed into the giant thick unsaturated zone in a large metal mining area in Yunnan. Based on hydrogeological conditions in the dewatering mining area, a vertical 2D-model of the 1 274 m level tunnel was properly built to simulate the migration process and groundwater environmental impact of Pb, while the backfill cracked. The results show that Pb migrates to the deep part with groundwater flow. The longest vertical migration distance and largest diffusion area are about 483.21 m and 0.51 km², respectively, at the 4 757^th d since the backfill was cracked. The hydraulic capture controls the diffusion of Pb. The backfill is one of the effective methods to reduce the risk of geological disasters and environmental problems in the metal mining areas.