The evolution of understory herbs is closely related to forest succession. In order to understand the characteristics and influencing factors of herb diversity in different succession stages of Picea crassifolia forest in the Qilian Mountains, this study adopted the method of space instead of time, and took the succession sequence composed of broad-leaved pure forest (Ⅰ) → broad-leaved mixed forest (Ⅱ) → coniferous broad-leaved mixed forest (Ⅲ) → Picea crassifolia forest (Ⅳ) as research objects. In order to provide theoretical basis for stand structure optimization and function improvement of Picea crassifolia forest in Qinghai province, the effects of biological factors and abiotic factors on herb diversity were analyzed. The results showed that:(1) a total of 81 species in 64 genera and 31 families were found under different succession stages, including 42 species in 33 genera and 21 families in broadleaf pure forest (Ⅰ), 40 genera and 50 species in 22 families in broadleaf mixed forest (Ⅱ), 51 species in 42 genera and 22 families in coniferous and broad-leaved mixed forest (Ⅲ), and 23 species in 21 genera and 15 families in Picea crassifolia forest (Ⅳ). (2) The species richness index, Simpson index, Shannon-Wiener index and Pielou index increased first and then decreased with succession, and the diversity indexes were the highest in the coniferous and broad-leaved mixed forest (Ⅲ) stage, and the lowest in the Picea crassifolia forest (Ⅳ) stage. (3) Among biological factors, leaf area index (LAI) was the leading factor affecting herb diversity, and stand density, diameter at breast height (DBH), tree height, and leaf area index were negatively correlated with herb diversity. (4) Among abiotic factors, soil organic matter and available potassium were the dominant factors, while soil moisture, available phosphorus, and alkali-hydrolyzed nitrogen were positively correlated with the diversity index. In conclusion, the understory herbaceous plants were the most abundant in the coniferous and broad mixed forest (Ⅲ), and LAI, available potassium and soil organic carbon were the main factors affecting the herbaceous diversity. When the Picea crassifolia forest with low herb diversity was reconstituted, the stand density could be adjusted appropriately and birch could be added to build the coniferous and broad mixed forest.