Abstract: It is important to investigate soil microbial biomass carbon, potential mineralized carbon and particulate organic carbon under different tillage patterns, mulching methods and fertilization regimes for evaluation of soil carbon pool change due to agricultural management. Based on a long-term mulching experiment in dry highland of the Losses Plateau, this study investigated dynamic changes of soil organic carbon at different crop growth stages under straw and plastic film mulching conditions. The aim of the study was to explore seasonal responses of soil organic carbon and its fractions to surface mulching and deepen the understanding of soil carbon cycle in dry farmland. The data were collected from the 8 years experiment with the treatments of straw mulching, plastic film mulching and no mulching (CK) at pre-sowing, seeding, jointing, belling-heading, filling and harvest stages of spring maize. The seasonal dynamics of soil organic carbon, microbial biomass carbon, potential mineralized carbon and particulate organic carbon in 0-10 cm, 10-20 cm and 20-40 cm layers were determined. The results showed that:1) the contents of soil organic carbon, microbial biomass carbon, potential mineralized carbon and particulate organic carbon showed a tendency of decreasing at seedling stage, increasing at jointing stage, decreasing again at belling-heading stage and recovering at filling and harvest stages. 2) Compared with the CK, straw mulching effectively increased the contents of soil organic carbon and its fractions at most growth stages of spring maize, which improved soil fertility and enhanced carbon sequestration. Plastic film mulching did not show significant effects at the early growth stages, but decreased soil organic carbon and its fractions contents at the later stages of spring maize. 3) The ratio of microbial biomass carbon to soil organic carbon did not change with growth stages among treatments. However, particulate organic carbon in 0-10 cm under straw mulching was significantly higher than that under CK and plastic film mulching treatments, indicating that it hugely contributed to the dynamics of soil organic carbon in the topsoil layer under straw mulching treatment. The dynamics of soil organic carbon under plastic film mulching were mainly attributed to changes in particulate organic carbon and potential mineralized carbon. 4) The proportions of potential mineralized carbon and particulate organic carbon decreased at belling-filling stage with plastic film mulching or without any form of mulching, but did not change throughout the growing season with straw mulching. This indicated that straw mulching recharged soil potential mineralized carbon and particulate organic carbon during later growth stages. In conclusion, straw mulching of spring maize in the Loess Plateau significantly increased soil organic carbon and its fractions contents with obvious seasonal variations, while plastic film mulching showed decreasing effect at later growth stages of spring maize.