The effect of forest gap (FG) on regeneration seedlings biomass accumulation and allocation is important for population recovery, especially in dark coniferous forests. The objectives of this study were to explore the effect of FG on accumulation and allocation process of root biomass for Abies faxoniana regeneration seedlings, and provide a reference for understanding of regeneration mechanism of A. faxoniana. The individual samples of A. faxoniana seedlings lived in the FG and under the forest canopy (FC) were harvested from the subalpine natural coniferous forest in western Sichuan and the roots were further divided into fine root (d<2 mm) and coarse root (d ≥ 2 mm). The fine root and coarse root biomass accumulations and allocations changed with basal diameter fitted by a power function as well as their allometric scaling relationships were modeled with standard major axis (SMA) regression. The results showed that the accumulations and allocations of root biomass for A. faxoniana seedling increased with the basal diameter growth, the fractions of roots biomass allocated to coarse roots increased gradually, while fractions allocated to fine roots decreased gradually, and their allometric relationships were adjusted from isometry scaling to allometric scaling. The median values of fine roots biomass, coarse roots biomass and total roots biomass in FG were lower than those in FC, and the fraction of fine roots biomass in FG was greater than those in FC. And the same time, FG significantly affected the accumulations and allocations process of root biomass in fine roots and coarse roots with the increasing basal diameter. The reduction of fraction of fine roots in FG was greater than that in FC, but the FG had minor effect on the allometric scale of between fine roots biomass and coarse roots biomass. Therefore, this study presented the evidence that FG could promote the A. faxoniana seedlings to adapt the FG via optimizing resource utilization such as underground-biomass allocation.