The metabolic activity of microbes profoundly influences underground ecological processes, similar to the soil carbon cycle. However, even though forest gap research has addressed underground processes, studies of microbial functional diversity remain scarce. In the present study, we mainly investigated changes in the microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and microbial functional diversity (AWCD) of the soil along a horizontal gradient of large, medium, and small forest gaps (radii of 4, 8, and 12m, respectively) in a plantation of Platycladus orientalis (L.) Franco in Xuzhou (China) during the summer. The corresponding gap sizes, which were calculated as the ratio of gap radius to the height of trees at the edge of the gap, were 1.5, 1, and 0.5, respectively. Three forest gap gradients, namely, forest gap edge (D1), 4m outside the gap (D2), and 8m outside the gap (D3), were chosen. After forest gap formation, soil MBC decreased on the whole and MBN decreased significantly (P<0.05), where as the MBC/MBN ratio increased significantly (P<0.05). Among the three gap types, the MBC and MBN were lowest in the large and small gaps, respectively, with minimum levels of 2069.42 and 126.21mg/kg. Meanwhile, the MBC/MBN ratio varied from 10.50 to 19.96 and generally decreased with increasing gap size. In addition, forest gaps reduced soil AWCD, and the effect size varied with gap size, with the smallest effects observed in the small gaps. From D1 to D3, the AWCD of the small gaps initially declined and then increased, whereas it continuously increased in the medium gaps and continuously decreased in the large gaps. Furthermore, forest gaps also reduced the carbon consumption of soil microbes, and the use of carbohydrates and amino acids was the lowest in the medium gaps, whereas that of polymers was the highest in the medium and small gaps. In D1, the soil microbes consumed less carbon in the medium gaps than in the small gaps. The present study also revealed that forest gaps improved microbial functional diversity. Although there were no significant differences between the diversity (H'), richness (S), and evenness (E) indices of the three gradients, the dominance index (D_s) values of D3 and D1 were significantly higher in the small gap and control plot and were significantly greater than those of D1 and D2 in the large gap (P<0.05). The effect of forest gap on the diversity indices varied with gap scale and location. The main carbon sources used by the soil microbes were polymers and amino acids. In short, the effect size of forest gap on soil MBC, MBN, and microbial functional diversity in P. orientalis plantations vary remarkably with scale. Therefore, forest gaps can promote soil carbon sequestration and macromolecular substances degradation and improve the ability of forest ecosystems to adapt to global climate change. Furthermore, medium gaps were the most efficient for improving the ecological function of forest plantations.