It is important to study the relationship between soil physicochemical properties and microbial biomass carbon(MBC)，microbial biomass nitrogen(MBN)，microbial biomass phosphorus(MBP)contents and their ecological stoichiometric characteristics of perennial alpine cultivated grassland under nitrogen addition，in order to maintain the temporary steady state of perennial alpine cultivated grassland. The mixed planting grassland of Poa pratensis Qinghai，Poa crymophila Qinghai and Festuca sinensis Qinghai，which was established in 2019，was used as the research object. CK(no nitrogen application)，U1(amide nitrogen 22.5 kg·hm-2)，U2(amide nitrogen 45 kg·hm-2)，U3(amide nitrogen 67.5 kg·hm-2)，A(ammonium nitrogen 67.5 kg·hm-2)and N(nitrate nitrogen 67.5 kg·hm-2)treatments were used to analyze the soil physicochemical properties，microbial biomass，and stoichiometric characteristics under different treatments. The results showed that different nitrogen forms and application levels had a significant impact on soil physicochemical properties and microbial biomass. With the increase of nitrogen application level，soil bulk density，pH，and microbial biomass carbon nitrogen ratio(MBC/MBN)gradually decreased，while soil available nutrients，soil total nutrients，MBC，MBN，microbial biomass carbon phosphorus ratio(MBC/MBP)，and microbial biomass nitrogen phosphorus ratio(MBN/MBP)gradually increased. Compared with different nitrogen forms at the same nitrogen level，ammonium nitrogen fertilizer had the most significant effect on reducing MBC/MBN，with a decrease of 15.61% compared to the CK；Nitrate nitrogen fertilizer had the most significant promoting effect on total nitrogen，MBC，MBC/MBP，and MBN/MNP. Redundancy analysis indicated that soil total nitrogen，pH and organic carbon were the sensitive indicator that affected soil microbial biomass and stoichiometric characteristics. Although nitrogen deposition promoted soil nutrient turnover and cycling in perennial alpine cultivated grasslands，the problems it brought to the ecological environment couldn’t be ignored.