Effects of straw returning directly and carbonization returning combined with nitrogen fertilizer on the content of carbon and nitrogen in soil and cotton yield were evaluated through a field positioning experiment for seven consecutive years. Using a two-factor experimental design, three straw returning modes were set up, which were no straw returning (CK), straw returning directly (ST) and straw carbonization returning (BC). Three nitrogen application rates were set up as 0 (N0), 300 (N300) and 450 (N450) kg/hm², respectively. The results showed that the content of soil organic carbon in ST and BC treatments increased year by year, and the increase of BC was greater than that of ST. In the 7th year, under different nitrogen application levels, the soil organic carbon of ST treatment was 33.28%~36.43% higher than that of CK, and that of BC treatment was 58.56%~63.25% higher than that of CK. Straw returning (ST and BC) for many years could improve the content of soil total nitrogen. Under N0 level, the highest of total nitrogen content was found in ST treatment. Under N300 level, the total nitrogen content of BC was significantly higher than that of ST after seven years. Under N450 level, the total nitrogen content of BC treatment was higher than that of ST after five years. Both ST and BC treatments increased soil C/N ratio under N0 level, while only BC treatment increased soil C/N ratio under N300 and N450 levels. Straw carbonization returning combined with nitrogen fertilizer application significantly increased cotton yield. Compared with CK, BC treatment increased cotton yield by 17.43%~17.89% under N300 and N450 levels. Therefore, straw carbonization returning combined with nitrogen fertilizer application could improve soil organic carbon content, nitrogen pool capacity and soil C/N ratio and cotton yield.