In karst area, the soil thickness is uneven and the soil habitats are highly heterogeneous. Shallow soil habitats with shallow wide soil are formed on hilly slopes and deep soil habitats with deep narrow soil are formed in the rock trenches. Availabilities of underground space and soil moisture are different in different habitats. The previous studies have focused on the ecological effects of soil thickness changes. However, the underground space is also very important for plant growth, even exceeding soil resources. Therefore, studying the effects of soil thickness on plants must consider the important roles of soil moisture and soil space. In this experiment, we used the perennial grass Lolium perenne L. as plant material. Simulating the soil characteristics in karst habitats, we set two kinds of soil containers including shallow and wide (SW:30×30×5 cm³) and deep and narrow (DN:10×10×45 cm³). Taking average daily precipitation in Chongqing for the past 30 years as the control water, three water treatments were control group (normal water conditions, W100%), mild water reduction (50% reduction, W50%), and heavy water reduction (70% reduction, W30%), respectively. The effects of soil moisture changes on the aboveground growth and photosynthetic physiology of L. perenne in two different soil habitats were explored to further understand the adaptation strategies of plants in karst area. The main results were as follows:(1) The SW habitats were sensitive to soil water changes, and soil water content in which decreased significantly with decreasing water application. Under mild water reduction, the growth of aboveground parts was promoted, mainly in the increase of leaf area and photosynthetic rate. However, the growth and photosynthetic physiology were inhibited under heavy water reduction, while the aboveground mass fraction and water use efficiency increased significantly. (2) The DN habitats had better water retention capacity, in which the soil water content decreased slowly with the water supply reduction. The relative leaf water content, stomatal conductance, and specific leaf area were not significantly different among three water treatments. However, the total biomass, the aboveground mass fraction, and plant water use efficiency all rebounded under heavy water reduction. Overall, the results suggested that plants in shallow and wide habitats tended to maintain higher photosynthetic rate by increasing the growth of aboveground parts, and distributing more biomass in the aboveground parts to cope with water stress; while plants in deep and narrow habitats sufficiently used soil space to promote root growth to increase water uptake for maintaining plants growth.