ObjectiveTo evaluate the correlation between level of specific IgM antibody of nucleocapsid protein (N protein) and severity of severe fever with thrombocytopenia syndrome (SFTS).MethodsClinical characteristics and laboratory detection results of 30 patients with SFTS in a tertiary firstclass hospital were analyzed retrospectively, clinical characteristics and prognosis between patients in N protein specific IgM(+) group and N protein specific IgM(-) group was compared; patients were divided into mild disease group and severe disease group according to the severity of disease, correlation between IgM antibody titer and RNA load of SFTS virus (SFTSV) in patients in mild disease group and severe disease group was compared; correlation between N protein specific IgM antibody titer, SFTSVRNA load, and severity of disease in patients were analyzed.ResultsThe numbers of patients with neurological symptoms, death, and serious disease in N protein specific IgM(-) group were significantly higher than IgM(+) group (all P<0.05); N protein specific IgM antibody level was negatively correlated with SFTSVRNA load, prothrombin time (PT), lactate dehydrogenase (LDH), and aspartate aminotransferase (AST) (r=-0.495, -0.440, -0.367, -0.280, respectively, all P<0.05); while was positively correlated with platelets (PLT) (r=0.335, P=0.002). SFTSVRNA load was positively correlated with PT, LDH, and AST (r=0.606, 0.604, 0.587, respectively, all P<0.001); while was negatively correlated with PLT（r=-0.384，P＜0.001）. On the 10-13 day of onset, the titer of N protein specific IgM antibody in patients in mild disease group was higher than that in patients in severe disease group (P<0.05).ConclusionThe presence and increase of N protein specific IgM antibody are helpful for clearance of SFTSV, and has a repairing effect on damage of coagulation function, liver and myocardial damage; N protein specific IgM antibody may be an important factor for predicting prognosis of patients.