To investigate the short-term effects of forest gap size on soil microbial biomass carbon and nitrogen in Platycladus orientalis plantations, we conducted a study in a 50-year P. orientalis plantation in Xuzhou. The effects of three levels of forest gap size (S, M, and L, with a radius of 4, 8, and 12m, respectively) and position (inside, edge, and outside the gap) on soil microbial biomass, carbon (MBC), and nitrogen (MBN) were analyzed two years after the creation of gaps. The results showed that (1) L gaps had significantly higher soil MBN content at the outside position in spring (207.1mg/kg) and summer (169.5mg/kg) than that in the control plots. The M gaps had significantly larger soil MBC content at the inside and edge positions (2959.3 and 3008.8mg/kg, respectively), soil MBN content at the edge position in spring (207.7mg/kg), and soil MBN content at the inside position (144.4mg/kg) in summer than those in the control plots. The S gaps had significantly reduced soil MBC (2159.2 and 1955.1mg/kg) and MBN (153.1 and 131.3mg/kg) content at the outside and edge positions in spring, respectively, than those in the control plots. (2) Soil MBC content was positively correlated with soil total organic carbon and dissolved organic carbon (DOC) content, but negatively correlated with soil temperature; soil MBN content was positively correlated with soil water content, DOC content, and soil total carbon. MBC content was not significantly correlated with MBN content in soil and litter. We recommend the M gap size as an efficient management practice for increasing soil microbial biomass, promoting soil carbon and nitrogen cycling, and improving soil fertility and tree growth in P. orientalis plantations.