The combined or mixed pollution of microplastics and antibiotics due to their same sources has become an emerging environmental problem in farmland soils. This study is to explore the impact of low-density polyethylene (LDPE) microplastics amendment on the degradation of two antibiotics:sulfamethoxazole (SMZ) and tetracycline (TC), the changes of ammonium and nitrate composition, the abundances of nitrogen functional genes (nifH, amoA, amoB, nirK and nirS) and two typical antibiotic resistance genes (sulI and tetA). The results showed that the degradation of SMZ and TC in the soil was not significantly changed after being amended with LDPE microplastics, however, the abundance of sulI and tetA resistance genes in the soil decreased significantly (P<0.05). A severer impact of LDPE microplastic amending was observed for ammonium nitrogen than nitrate nitrogen in the soil. LDPE microplastics amended with 0.05% (m/m) significantly (P<0.05) increased the content of ammonium nitrogen, and such an impact was most prominent on the 15^th day of incubation. LDPE microplastics significantly decreased the abundance of nifH, nirK and nirS genes and changed the composition of the functional genes of the ammonia oxidation process (amoA and amoB). The abundance difference between amoA and amoB decreased with the increase of microplastics amendment. This study is of great significance for further understanding the changes in nitrogen form and the risk of resistance genes in soil environment under a microplastics-antibiotics composite/mixed pollution system.