The effects of plastic film mulching on soil organic carbon and nitrogen fractions were investigated in a spring maize field. Three treatments, i.e. plastic film mulching, no mulching, and bareland fallow were included and soil organic carbon (SOC), total nitrogen (STN), particulate organic carbon and nitrogen (POC, PON), potential mineralized carbon and nitrogen (PCM, PNM), and microbial biomass carbon and nitrogen (MBC, MBN) were measured at 0-40 cm soil depth. No significant differences in SOC and STN were found among treatments for all soil depths. Compared with no mulching, the contents of POC, PON, POC/SOC, and PON/STN at 0-40 cm soil depth was decreased by 29.0%, 33.3%, 29.9%, and 35.7% by plastic film mulching, respectively. The PCM content and PCM/SOC was decreased by 17.8% and 16.1%, respectively, and the MBN content and MBN/STN at 0-10 cm soil depth was increased by 10.6% and 10.5% (p < 0.05), respectively. Compared with the bareland fallow, the content of PCM and PNM at 0-40 cm soil depth was increased by 12.8% and 14.7 by no mulching, respectively, and was increased by 7.8% and 6.5% (p < 0.05) by plastic film mulching, respectively. The contents of MBC and MBN and their contributions decreased after maize planting in spite of plastic film mulching or not. In general, plastic film mulching could increase microbial biomass carbon and nitrogen at the top layers but decrease both the contents and conributions of moderate labile fractions, indicating that plastic film mulching may not be beneficial for long-term soil carbon and nitrogen sequestration in dryland maize fields.