Abstract: Water accumulation is widely present in the goaf of coal mine in China. It is of great significance to clarify the water-rock interaction mechanism in the closed and semi-closed goaf environment for water pollution reduction and groundwater resource protection in coal mine area. Taking the water accumulation in the goaf of a coal mine in Inner Mongolia as the research object, we designed a box simulation test to highly restore the goaf environment. X-ray diffractometer (XRD) and X-ray fluorescence spectrometer (XRF) were used to analyze the mineral composition and element composition of the simulated filler of goaf (coal and caving rock mass). The hydrochemical characteristics of water were characterized with the aid of ion chromatograph (IC) and the inductively coupled plasma optical emission spectrometry (ICP-OES). The water-rock interaction mechanism of water-quality formation for accumulated water in the goaf was explored. The results show that the water-rock interactions mainly include the mineral dissolution-precipitation and cation exchange in coal and caving rock mass, as well as the pyrite oxidation and mixing in coal. Therein, mineral dissolution is dominant. Na⁺ and K⁺ in water accumulated in goaf are mainly derived from the dissolution of silicate minerals such as albite and potassium feldspar, while Ca²⁺ and Mg²⁺ are mainly from the dissolution of calcium feldspar, calcite and chlorite. Cation exchange mainly occurs in the initial stage (0-20 d) of water-rock interaction in goaf, and then gradually weakens. The oxidation of pyrite in residual coal at the bottom of the goaf is the main reason for the increase of SO₄ ²⁻ concentration, the enhancement of reducibility, and the reduction of pH and Dissolved Oxygen (DO). Besides, the mixing effect gradually became prominent in the middle and late period (30-150 d) of the test, which made the hydrochemical characteristics of the groundwater accumulated in goaf tend to be consistent. Generally, the results of the study provide theoretical references for the prediction and prevention of accumulated groundwater pollution in coal mining goaf.