Objective: This study aimed to explore the diagnostic value of serum severe acute respiratory syndrome coronavirus 2 (SARSCoV2) nucleocapsid(N) protein assay in the early stage of SARSCOV2 infection. Method: Serum N protein in SARSCOV2 infected patients and non SARSCOV2 infected population was measured by enzyme-linked immunosorbent assay (ELISA) double antibody sandwich assay. Colloidal gold immunochromatography assay is used to detect serum N protein antibodies in the above population. Results: 50 cases of SARSCoV2 nucleic acid positive and SARSCoV2 antibody negative patients had a serum N protein positive rate of 76%, including 2% with a concentration of 10.00~49.99 pg / mL, 8% with a concentration of 50.00~99.99 pg / mL, 22% with a concentration of 100.00 ~299.99 pg/mL, and 44% with a concentration ≥300.00 pg / mL. 37 samples of patients with serum SARSCoV2 antibody positive after infection had a serum SARSCoV2 N protein positive rate of 2.7%, of which 2.7% had the concentration of 10.00~49.99 pg / mL and 0% had the concentration of 50.00~99.99 pg / mL, 100.00 ~299.99 pg / mL, and ≥300.00 pg / mL. Serum N protein test results of 633 non SARSCOV2 infected patients including pregnant women, other respiratory infections, and increased rheumatoid factor were all negative, having a serum N protein concentration less than 10.00 pg/mL, with a specificity of 100%. Using SPSS 19.0 to calculate the receiver operating characteristic curve, the area under the curve was 0.9756 (95% confidence interval 0.9485~1.000, p <0.0001), sensitivity and specificity were 92% (95% confidence interval 81.16% to 96.85%) and 96.84% (95% confidence interval 95.17% to 97.15%). The best CUTOFF value is 1.850 pg / mL. Conclusion: The measurement of SARSCOV2 serum N protein has a high diagnostic value for the infected patients before the antibody appears, and shortens the window period of serological diagnosis. The laboratory needs to establish an individual CUTOFF value according to purpose of the application.