Mae Moh mine is a large open pit coal mine located in Lampang province, Thailand. The mine has been operated by EGAT since 1955 and supplies up to 45,000 tons of lignite daily to feed EGAT’s power plant. Groundwater is the common problems for mine operation. Poorly control of groundwater movement will have negative impact on the safety, efficiency and economic situation of mining. This research is aimed to simulate groundwater system in mining area for groundwater management to help prevent the problems of pit wall instability and floor heave from groundwater pressure. The groundwater model uses the 3D GMS^® software which is the finite difference groundwater flow model to simulate the groundwater system. The geological, hydrological, hydrogeological data are collected and analyzed before putting into the conceptual model. After that, the conceptual model is translated into numerical model, the model simulates in transient state and calibration runs. The calibration process performed until the piezometric levels from the model are consistent with the results from observation wells measurements. The slope stability in C1 west wall pit is analyzed. The floor heave is evaluated as well. The instability of slope and floor heave conditions are defined. The calibrated model is used to predict the range of depletion of groundwater table. The calibration results show that the accuracy of groundwater flow model is 98.64%; the comparison of contour lines between measured heads and calculated heads provides good results. The simulation results suggest that, in order to ensure safe mining from slope instability and floor heave, the dewatering requirements from 2021 to 2049 are at an average of 5,153 cubic meters per day. With this prediction, the groundwater level will be depressed to be lower than the lowest pit floor, on a yearly basis, until the final stage plan in year 2049.