Montane secondary forests played an important role in biodiversity conservation. Changes of vegetation species diversity in understory during the development of secondary forest could directly affect ecosystem stability. Therefore, revealing the changes of vegetation species diversity in understory of secondary forests at different developmental stages, as well as their influencing factors and applicable diversity indices was significant for the conservation and the assessment of species diversity. In this study, we selected seven α-diversity indices and two β-diversity indices for the four developmental sequences of secondary forest, community establishment stage (I)-competitive growth stage (II)-quality-selective stage (III)-near-natural stage (IV) in the mountains of Northern Hebei Province. We aimed to provide theoretical basis for the conservation and assessment of species diversity in the region. The results showed that: (1) The α-diversity indices of the shrub layer showed a general trend of decreasing-rising-decreasing during the secondary forest development; the species diversity indexes and species evenness indexes of the herb layer showed a decreasing and then an increasing trend, while the species richness indexes showed a decreasing-rising-decreasing trend. (2) With the development of the secondary forest, the differences in species composition were decreasing, while the differences in the relative abundance of shared species were increasing, both the Jaccard index and the Chao index first decreased and then increased. This indicates that the understory vegetation underwent a dual process of species reorganization and fluctuations in the relative abundance of shared species. (3) Soil pH, mixing degree, soil organic matter and soil water content had significant effects on species diversity in the shrub layer, while average tree height, average diameter at breast height and soil total phosphorus content had significant effects on species diversity in the herb layer. (4) The contribution of species evenness to the community species diversity index was higher than that of species richness. (5) The most suitable index for assessing species richness of vegetation in understory of secondary forests at different developmental stages was the Margalef index, the species diversity index was the Shannon-Wiener index, and the species evenness index was the Pielou index. The results indicated that soil physicochemical factors had a dominant role in the species diversity of the shrub layer in the understory of different developmental secondary forests in the mountain area of Northern Hebei Province. However, stand structure factors had a dominant role in the species diversity of the herb layer. This study provided theoretical support for biodiversity conservation and assessment in this region.