In order to explore the effect of biodegradable polylactic acid microplastics (PLA-MPs) on nitrogen conversion in sediments, a laboratory experimental sediment system was constructed, and 0 (control), 0.05%, 0.5% and 5% (W/W) PLA-MPs were added to freshwater sediments, then the incubation experiment was performed for 45d at 25℃ and light intensity of 40µE/(m²·s). The concentration of dissolved organic carbon (DOC) decreased significantly and the concentration of dissolved organic carbon (DOC)increased significantly (P<0.05), and the formation of CO₂ and CH₄ was promoted. At the end of the experiment, the concentration of NH₄⁺-N was reduced by the addition of PLA-MPs, and the concentrations of NO₃^--N and TN in the 0.05% and 0.5% PLA-MPs treatment group were lower than those in the control group (compared with the control group, the TN concentrations of the overlying water decreased by 68.44% and 61.83%, respectively). On the contrary, the TN accumulation in the 5% PLA-MPs treatment group was recorded (the TN concentration in the overlying water was 5.71mg/L at the end of the experiment) and was significantly higher than that in the control group (P<0.05). The NO₂^--N concentration in the 0.5% and 5% PLA-MPs treatment groups decreased and the release of N₂O was reduced, while the concentration of NO₂^--N in the 0.05% and 5% PLA-MPs treatment groups was increased. The addition of PLA-MPs promoted the expression of nitrogen-fixing genes nifH and nitrification genes amoA, and the denitrification genes nirS and nosZ were enriched in the 0.05% and 0.5% PLA-MPs treatment groups. However, the abundance of denitrification gene narG was only up-regulated in the 0.5% PLA-MPs treatment group. The abundance of narG and nirS genes in the 5%PLA-MPs treatment group was down-regulated, and the expression of nosZ gene was inhibited but then promoted. The results show that PLA-MPs changes the properties of the overlying water and sediment, promotes nitrogen fixation and nitrification, and provide carbon source to enhance denitrification and denitrification under 0.05% and 0.5% PLA-MPs treatments, but the reduction of NO₃^--N and NO₂^--N is inhibited due to low pH in 5% PLA-MPs treatment, resulting in TN accumulation.