Congenitally slow soil development rates and acquired rocky desertification ensure a serious shortage of soil resources in karst areas. Coupled with the steep terrain, soil distribution is uneven with a high degree of heterogeneity in space in karst regions. In the karst depression, basin, and valley areas, soil thickness and soil distribution are continuous; however, in the hill slope area, soil is usually "rock soil" or is missing. Uneven distribution of karst soil also leads to obvious spatial heterogeneity of moisture. Global precipitation changes might aggravate heterogeneity of soil moisture in soils of different thicknesses. Plant morphogenesis is very sensitive to soil moisture, including the leaf veins that control water transport and utilization and the stomata that adjust the balance of water and CO₂. Simultaneously, photosynthesis that utilizes water and CO₂ as raw materials is also directly affected by soil moisture. Thus, exploring the responses of the leaf anatomical structure and photosynthetic traits of plants to water availability and soil heterogeneity in karst regions is necessary to understand how precipitation changes might affect plant growth and physiology in soils of different thicknesses. Herbaceous plants, especially grasses in karst regions, are most often affected by soil heterogeneity and water availability because they mainly utilize water and nutrients from the surface soil via their fibrous root system. Two graminaceous perennial grasses, Lolium perenne L. and Festuca arundinacea Schreb. were chosen for the present study. These two species are often chosen as pioneer plants for ecological restoration and reconstruction in karst regions because of their attributes of fast growth, strong adaptive ability, and high yield, which effectively promote economic development and assist to alleviate rural poverty in harsh karst regions. In the present study, three water treatments (W_CK:40 mL/d, D1:20 mL/d, and D2:12 mL/d) were combined with three levels of soil thickness[shallow soil (S_S:5 cm), control (S_CK:15 cm), and deep soil (S_D:30 cm)] in a factorial randomized design and measurements were obtained of leaf vein and stomata traits and several photosynthetic parameters. The following results were obtained:(1) Under normal water supply conditions (W_CK), the stomatal density and stomatal limitation (Ls) of L. perenne and F. arundinacea were significantly higher in the S_S group than in the S_CK group, and the two species net photosynthetic rate (Pn), intercellular CO₂ concentration (Ci), and transpiration rate (Tr) decreased. In the S_D group, the stomatal density of both species decreased, the leaf density, Pn, and Tr of L. perenne were lower than those in S_CK, and the leaf density and Pn of F. arundinacea increased. (2) Under D1 water treatment, the stomatal density of L. perenne in S_S was higher than that in S_CK, and leaf vein, Pn, and Tr decreased; however, the stomatal density of F. arundinacea in S_Sshowed some decrease, and the leaf vein, Pn, and Tr of F. arundinacea were not significantly affected. In the S_D group, the stomatal density of L. perenne did not change, the leaf vein increased, and the Pn and Tr were both decreased, while the stomatal density of F. arundinacea decreased and the leaf vein, Pn, and Tr increased. (3) Under D2 water treatment, the leaf veins of the two species increased in the S_S group as compared to that in S_CK, and their stomatal density, Pn, and Tr were suppressed. In the S_D group, compared to S_CK, the stomatal density of L. perenne decreased and the other parameters in both plants were not significantly affected. The results of the present study indicate that under different water conditions, the leaf anatomical structure and photosynthetic characteristics of the two plant species studied were different in response to the different thickness soils, with differences recorded between the different species. Overall, as the water content decreased, the inhibition that plant photosynthesis suffered was enhanced by a decrease of soil thickness, and the promotion effect that the deep root plant received and the inhibition that the shallow root plant suffered were first strengthened and then weakened by an increase in soil thickness. The variation of vein density and stomatal density with water content is related somewhat to the change of leaf area and leaf width.