Fragmented landscapes are mosaics of vegetation patches with differing proportions. A more comprehensive understanding of the effect of landscape structure characteristics on species diversity is necessary to improve conservation strategies for species diversity in fragmented landscapes. Sampling plots were established in 38 natural vegetation patches within the Saihanba Nature Reserve, including 12 herbs, 11 shrubs, and 15 natural forest vegetation fragments. Herbaceous plant species were divided into C₃ and C₄ functional groups according to their photosynthetic carbon-assimilating pathway. The patch area, shape index, isolation index, and percentage of forest and grassland within a 500 m radius from the boundary of each sampling patch were selected as the landscape factors. A structural equation model (SEM) was adopted to explore the relationships between species richness of functional groups (C₃ and C₄) and landscape factors. Patch area (2.18-74.06 hm²) had a significant positive effect on C₃, C₄ herbs, and total species richness (P < 0.05), and the effect on C₃ herbs was the highest; the shape (1.06-3.11) and isolation (33.51-327.65) indexes had no significant effect on C₃, C₄ herbs and total species richness (P > 0.05); the species richness of C₃, C₄ and all herbs increased with increasing grassland proportions (4.20%-64.95%) in the surrounding matrix, whereas the percentage of forest (35.05%-95.80%) in the surrounding matrix did not have a direct effect. Patch area and vegetation composition of adjacent patches were the main factors affecting the C₃ and C₄ herbs in Saihanba Nature Reserve. Preserving the large natural vegetation patch and improving the proportion of grassland in the surrounding matrix could effectively conserve key C₃ and C₄ herbaceous plant species.