[Objective] In order to quantitatively evaluate the suitability and quality improvement potential of forest and grass in typical watersheds of complex underlying surface in the middle reaches of the Yellow River. [Methods] The evaluation index system and evaluation method of forest and grass suitability in Sanchuan River Basin were constructed by means of geographical detector, spatial analysis, mathematical statistics and other methods, and the forest and grass suitability and its potential for quality improvement under the current land use conditions in the basin were evaluated. [Results] (1) The dominant factors affecting the suitable distribution of forest land in the Sanchuan River Basin include the fraction of absorbed photosynthetically active radiation (FPAR), elevation, soil erosion intensity and soil type. The dominant factors affecting the suitable distribution of grass land include elevation, FPAR, soil type and soil moisture, etc. (2) The area of forest land with different suitability degrees was from large to small high suitability, unsuitable, mid-height suitability, moderate suitability, and the area above moderate suitable accounted for 81.9% of the total area of forest land. The area of grassland with different suitability degree was in order from large to small: moderate suitability, unsuitable, mid-height suitability, and high suitability, and area above moderate suitability accounts for 55.3% of the total area of grassland, and there was still a large space for optimization layout of grassland in the basin. (3) The area of forest land improvement was 223.2 km², accounting for 15.4% of the total area of moderate suitability and above areas, and most of the forest land in the area had reached the coverage threshold level. The area of grade I upgrading potential of forest in each suitability area accounted for more than 89.0%, and the difficulty of upgrading was low. The area of grassland improvement was 222.3 km², accounting for 43.1% of the total area of moderate suitability and above areas, and nearly half of the grassland in the area have not yet reached the coverage threshold; The area proportion of grade I upgrading potential of grassland in each suitability area decreased with the increase of suitability, and the difficulty of upgrading increased with the increase of suitability. (4) The forest land can implement weak intervention measures based on natural restoration by strengthening the control of closure; grassland needs to take appropriate intervention measures according to the potential zoning of different suitability to achieve the overall improvement of grassland coverage. [Conclusion] In the Sanchuan River Basin, the pattern of forest land was better than that of grassland and the potential of grassland improvement is higher than that of forest land. The evaluation method of forest and grass habitat suitability constructed in this study can provide a scientific basis for the rational allocation of plant measures and the improvement of quality and efficiency in the middle reaches of the Yellow River.