China is a large country of coal production and consumption. The reserves and outputs of the thick coal seam (coal thickness more than 3.5m) both amount to 45% of the total amount, and 95% of the output comes from the underground mines. The longwall top-coal caving technology (LTCT) was generalized in China at the beginning of the 1980's, and now the LTCT has grown up and occupied an important position in thick coal seam mining. In China the soft coal seam accounts for quite a lot of the thick coal seam. The paper mainly studies the working coal face stability of longwall top-coal caving in the soft coal seam by the distinct element numerical simulation software UDEC2D. The coal-cutting height is divided into five cases: 1.8m,2.0m, 2.2m, 2.5m, 2.8m and 3.2m. The results show that the limit coal-cutting height is 2.2m in the soft coal seam. Under this height, the movement behaviours of the overlying rock stratra for the different coal thickness are studied. The research conclusions are important to the practical mine production.