The effects of subtropical Moso bamboo expansion on carbon accumulation in soil microbial residues in Chinese fir forests and its mechanisms were not clear. In this study, the litter (O layer) and soils of different pedogenic horizons (A horizon, B horizon, BC horizon) in the expansion zone of Moso bamboo to Chinese fir forest (including Chinese fir forest, Chinese fir and Moso bamboo mixed forest, and Moso bamboo forest) were studied. We analyzed the amino sugar content in litter and soil samples to characterize accumulation of the microbial residual carbon and further evaluated the role of microorganisms in soil organic carbon (SOC) formation. The results showed that Moso bamboo expansion significantly reduced the amount and carbon content of litter in Chinese fir forests, but significantly increased the content of fungal residual carbon (MRC-f), bacterial residual carbon (MRC-b), and total microbial residual carbon (MRC). Moso bamboo expansion significantly increased SOC, MRC-f and MRC-b and MRC in Chinese fir forests, and the proportion of MRC-f and MRC-b and MRC in SOC increased significantly at the beginning of Moso bamboo expansion (when Chinese fir forests evolved into Chinese fir and Moso bamboo mixed forest). These results indicate that Moso bamboo expansion enhanced the cumulative effect of soil MRC in Chinese fir forests and also increased the microbial contribution to SOC. However, in the late stage of Moso bamboo expansion, the proportion of MRC-f, MRC-b, and MRC to SOC did not change significantly, implying that the contribution of MRC and plant-derived carbon to the SOC content increased at the later stage of Moso bamboo expansion, and the relative proportion of their contribution remained unchanged. The content of MRC gradually decreased with soil depth, while the proportion of MRC to SOC gradually increased with the depth of the soil, implying that the microbial contribution to SOC was higher in deep soils than in surface soils. The results of the study have important theoretical implications for understanding the role of microorganisms in SOC formation in subtropical forests, scientifically assessing the impact of Moso bamboo expansion on the subtropical forest ecosystems and climate change, and adopting reasonable forestry measures to enhance the soil carbon sink function in subtropical forests.