Quercus aliena Blume var. acuteserrata Maxim. ex Wenz. is an important dominant species in temperate mid-elevation mountainous areas of China with significant functions in water resource conservation and ecological stabilization in forested ecosystems. Forty-two plots, including 210 subplots, were used to document the occurrence of trees, saplings, woody seedlings and habitat characteristics for this tree and associated woody species in a second growth forest in the Qinling Mountains in 2010. We analyzed the important values of trees, saplings and woody seedlings, regeneration niche breath, and tree regeneration dynamics under different conditions, including stand density, aspect, and altitude in Q. aliena var. acuteserrata secondary forests. The results show Q. aliena var. acuteserrata had an important value of 149.18% with none of the other 24 woody tree species in the tree layer approaching that level of importance. The subdominant species in the tree layer were Pinus armandii Franch. and Carpinus cordata Blume. 25 tree species of the 41 woody species in the regeneration layer. The woody plants in the regeneration layer were abundant with trees seedlings dominating. This appears to be beneficial to the optimization of the community structure and function. The dominant seedling species were Acer grosseri Pax, Litsea pungens Hemsl., and others. Seedlings of these two species were abundant in the regeneration layer with their height and age class lower than the dominant species. The height class of seedlings and saplings in the understory of Q. aliena var. acuteserrata secondary forest shows that woody plants regenerated vigorously. The biological chain of seedling to sapling was complete. Most regeneration was from seedlings rather than stump sprouts. We analyzed ten kinds of dominant regeneration tree populations. The regeneration niche breath of the sapling population was higher than that of seedling population for the same tree species which indicated the saplings has the broader use of community resources. The niche breath sequence from high to low seedling population was: Corylus heterophylla Fisch. ex Trautv. > Q. aliena var. acuteserrata > A. grosseri > other species. For saplings it was Corylus heterophylla > A. grosseri > Q. aliena var. acuteserrata > other species. Stand density had different effects on the density of saplings and seedlings. Sapling and seedling density increased with the stand density from 720 trees/ha to 1460 trees/ha, then decreased with the stand density increasing. The saplings density decreased continuously as the stand density increased. The direction of slope had various effects on the seedlings and saplings. A sunny SW facing slope of 3° was the most beneficial to seedlings and saplings. As the slope changed direction to create a shady slope, the seedlings density increased and the sapling density decreased. Sapling and seedling density changed with altitude. The saplings density increased as altitude increased from 1083 m to 1547 m, then decreasing with altitude up to 1882 m. The seedlings density increased from lower altitude to higher altitude.