In view of the discharge quantity of taihngs is great, the capacity of railings reservoir is insufficient and land acquisition is difficult for building new railings reservoir in the mine of transition from open pit to underground in China, the technical project of using open mined area for discharging railings is brought forward. The authors adopt finite element analysis software - ANSYS to simulate the railings discharge into the south of open mined - out area while underground mining is made in the north area in Chengjiagou Iron Mine, analyze the stress and strain of stope, isolation layer and rock slope of railings reservoir during mining ore, and suggest appropriate precautions against leakage. The result indicates that it is practical to use the open mined - out area for discharging railings and can ensure safe in production of the mine. The results of the study are of practical guidance significance for reducing the land used for tailings reservoir, saving construction investment and maintenance cost of railings reservoir and decreasing environmental pollution caused by railings.