The research area lies on the edge of the Guoliji Monocline, in which many water source field have been built up, and the source water supply is just match to the need at present, so it is very important to assess the exploited quantity for the new water source field in the Tangcun — Xilong River area. Based on the analyses of the geohydrologic condition in the research area and the vicinity, setting the mine projects according to the normal and low flow years, applying the software of numerical simulation of groundwater (Visual MODFLOW), numerical simulation of water resource of karstic water source field in the Tangcun — Xilong River area was carried out; thereafter recognizing and verfying the models, and making the rational evaluation about minable quantity of the new water source field in the Tangcun — Xilong River area, simultaneously comparing three mine projects, and finally determining the third mine project. In view of considering the subsistent problem about well lose at the stage of the constraint to water level, so the result would be more objective and practical. The result of the simulation could be a reference to the minable quantity of karstic water source field.