Abstract: To explore the effect of gramineous green manure on soil carbon and nitrogen contents in saline alkali cotton fields, two low-temperature- and saline alkali-tolerant gramineous green manures, ryegrass ‘Dongmu 70’ and barley ‘Zhudamai No.4’ were selected for in situ returning experiments from 2018 to 2019. Three treatments were set up: winter fallow farmland-cotton (T1), ryegrass-cotton (T2), and barley-cotton (T3). The contents of soil organic carbon (SOC), soil total nitrogen (TN), soil microbial biomass carbon (SMBC), and soil microbial biomass nitrogen (SMBN) were measured in different treatments at different periods (15, 50, 110, and 180 d) after returning green manure to field. The soil microbial quotient (SMQ) and ratio of soil microbial biomass carbon to nitrogen (SMBC/SMBN) were calculated. The results showed that both T2 and T3 significantly increased the contents of SOC and TN, and reached maximum values of 9.50 g∙kg⁻¹ and 798.84 mg∙kg⁻¹ (T2) and 9.91g∙kg⁻¹ and 759.34 mg∙kg⁻¹ (T3) at 180 d after returning green manure, respectively, and they were significantly higher than those of T1 treatment by 29.60% and 27.85% (T2) and 35.20% and 25.13% (T3), respectively. The variation dynamics of SMBC and SMBN contents in T2 and T3 were basically similar throughout the returning period, indicating a trend of stable growth in the early stage and significantly higher contents than those in T1, and a decrease in the latter stage and slightly lower contents than those in T1 at 110 d. The maximum values of SMBC and SMBN were 217.84 mg∙kg⁻¹ and 34.51 mg∙kg⁻¹ for T2, and 212.88 mg∙kg⁻¹ and 33.43 mg∙kg⁻¹ for T3 at 50 d and were higher than T1 by 81.46% and 47.76%, and 77.33% and 43.13%, respectively. In addition, the contents of SMBC and SMBN at different periods after returning to the field demonstrated that T2 was higher than T3. The change trend in SMQ in different treatments was consistent with that of SMBC. The two green manure treatments showed higher SMQ except for 110 d. T2 reached a maximum value of 2.82% at 15 d, while T3 reached a maximum value of 2.98% at 50 d. The SMBC/SMBN values of each treatment varied from 4 to 7; therefore, the microbial community in the soil was concluded to be mainly bacteria after returning the green manure to the field. T2 and T3 showed higher SMBC/SMBN values compared with T1, except at 110 d. In conclusion, the planting and return to the field of gramineous green manure in winter fallow farmland in saline alkali soil can significantly improve soil carbon and nitrogen contents in cotton fields, ameliorate the composition of soil microbial communities, improve the effect of soil microbial carbon sequestration, and provide nutrients for the growth of subsequent crops. The research results have guiding significance for the rational utilization of winter fallow farmlands in saline-alkali soils.