Objective：to investigate the dynamic expression of serum miRNA-21and inflammatory cytokine PCT in the course of sepsis，and to investigate its relationships with the severity and prognosis of sepsis. Methods：A total of 58 patients with sepsis and 24 healthy controls were collected，including 32 cases of general sepsis and 26 cases of septic shock.All patients with sepsis were divided into the survival group（n=48） and the death group（n=10） by the 28 day mortality rate. Serum miRNA-21 expressions and PCT were de－tected on 1，3，5 days after admission，and the basic clinical information was recorded. Results：①Compared with those of healthy controls，the serum miRNA-21 level of 1 day after admission in patients with sepsis was significantly higher（Z=-7.061，P=0.000），and the expression of miRNA-21of 1 day was valuable in sepsis diagnosis by ROC curve（area under the curve=0.997，95%CI=0.991 to 1.000，P=0.000）.Serum miRNA-21 more than 0.875 was taken as the cut-off point in the diagnosis of sepsis，and the sensitivity and specificity were 100% and 91.7%. ②In sepsis 5 d course，dif－ferent time points of miRNA-21expression differences were sta－tistically significant（F=67.778，P=0.000），and statistically significant differences in two different time points were ob－served（P=0.000，P=0.000，P=0.000）. Serum miRNA-21 expres－sion of 1 day after admission （1.974±0.886） ng/μL was higher than miRNA-21 expression of 3 day after admission（1.015±0.709） ng/μL；serum miRNA-21 expression of 3 day after admission （1.015±0.709） ng/μL was higher than miRNA-21 expression of 5 day after admission（0.531±0.312） ng/μL. ③In sepsis 5 d course，different time points of PCT expression differences were statistically significant（F=34.304，P=0.000），and statistically significant dif－ferences in two different time points were observed（P=0.000，P=0.000，P=0.000）. Serum PCT expression of 1 day after admission（14.164±13.939） μg/L was higher than PCT expression of 3 day after admission（4.224±4.039） μg/L；serum PCT expression of 3 day after admission（4.224±4.039） μg/L was higher than PCT expression of 5 day after admission（1.614±3.438） μg/L.④In septic shock group，the serum miRNA-21 expression of 1 day，PCT and APACHEⅡ were higher than those in general sepsis group（Z= -6.510，P=0.000；Z=-4.457，P=0.000；Z=-6.517，P=0.000）. The expression of miRNA-21 was positively correlated with APACHEⅡ score（r=0.993，P=0.000）. ⑤The expression of serum miRNA-21 in death group was higher than that in survival group，and there were significant differences between the two groups at each time point（Z=-2.823，P=0.005；Z=-3.664，P=0.000；Z=-4.942，P=0.000）. Conclusion：Serum miRNA-21 may be a potential marker for early diagnosis of sepsis，and has positive clinical signifi－cance in predicting the severity of sepsis and evaluating the prognosis. The dynamic expression of serum miRNA-21 in the course of sepsis may be closely related to the inflammatory response of the body.