Forest spatial structure has a significant influence on the regeneration of saplings. To clarify the key factors affecting the regeneration of saplings, the natural secondary forest of Cyclobalanopsis glauca forest was taken as the research object in Hunan Qingyang Lake State-owned Forest Farm. The density, tree height, ground diameter and crown width of saplings were measured, and the mixing degree, size ratio, opening index and aggregation index were calculated to represent forest spatial structure. The regeneration of saplings under the forest was evaluated and the correlation between them was explored by using the grey relational analysis method. The results show that there are 6 species of undergrowth regeneration saplings in 13 sample plots of C. glauca forest in Hunan Qingyang Lake State-owned Forest Farm, belonging to 4 families and 5 genera, which are C. glauca, Castanopsis sclerophylla, C. multinervis, Choerospondias axillaris, Dalbergia hupeana and Pinus massoniana. C. glauca is the dominant species in regeneration saplings, and the importance value of saplings is 63.09%. The spatial structure index and undergrowth regeneration index of each forest spatial structure of C. glauca are greater than 0.5, indicating a high correlation degree. Based on the mean correlation degree of all forest spatial structure indexes to undergrowth regeneration index, the order of influence on undergrowth regeneration is size ratio> openness> aggregation index> mixing degree, and size ratio has the most significant influence on undergrowth regeneration of C. glauca. Therefore, to promote undergrowth regeneration of natural secondary forests of C. glauca, we should adjust the size differentiation degree of trees, and consider the spatial distribution pattern of trees comprehensively.