[Objective] To investigate the optimal density of Mongolian pine(Pinus sylvestris var. mongolica) plantation for sand fixation in order to provide reference for the analysis of dynamic water balance, decline causes and stand stability evaluation in the study area.[Methods] We took the Mongolia pine filed in the southern edge of Horqin sandy land, Zhanggutai, Liaoning Province as a case study to monitor soil moisture variation(0-200 cm), rainfall in growing season and leakage below 200 cm. The surveys were conducted in three different Mongolia pine fields with various plantation densities(400 plants/hm², 600 plants/hm² and 800 plants/hm²).[Results] The temporal and spatial variation trends of soil moisture in all the three plantation densities were consistent. Soil layer at 0-30 cm was influenced by rainfall dramatically, while soil layer at 60-200 cm was only influenced by rainfall >40 mm. Soil moisture regression processes dissipated from lysimeter type to evaporation-leakage type after rainfall. Soil moisture showed significant differences among three plantation densities(p<0.05) in topsoil(0-200 cm):400 plants/hm² >600 plants/hm² >800 plants/hm². Water leakage was observed in all three plantation densities below 200 cm. The 800 plants/hm² field had the least leakage.[Conclusion] In wet years, water balance can be maintained in all three densities, while water deficit and recession may occur in normal year and years with extreme precipitation. The optimal planting density of Mongolian pine is around 400 plants/hm² in the sandy land.