Background: Coronavirus disease 2019 (COVID-19) has spread rapidly in China and globally. In order to control the spread of the epidemic, it is important to find an efficient diagnostic method. Objectives: The aim of this study was to assess the responses of antibodies during SARS-CoV-2 infection in relation to disease severity and to evaluate the association between the positive rate of antibody detection and nucleic acid test. Methods: Ninety patients with SARS-CoV-2 infection were recruited in this retrospective observational study. Demographic, clinical data, and SARS-CoV-2 IgM and IgG antibodies in serum specimens were detected at 4 and 6 weeks after diagnosis. Results: IgM and IgG antibody levels showed a decreased tendency, the titers at week 4 were higher than the titers at week 6: The positive rates of IgM at week 4 and 6 were 92.9% and 67.9%, respectively. The positive rates of IgG at week 4 and week 6 were 100%. No association was found between the positive rate of antibody detection at week 4 or 6 and that of nucleic acid test (P>0.05). No difference between the positive rate of antibodies against SARS-CoV-2 in severe and non-severe COVID-19 patients was observed. Conclusions: Antibody detection is an effective means in the diagnosis of COVID-19. The titer and positive rate of IgM are lower than those of IgG in the first six weeks after infection. Positive rate of antibodies was not different between severe and non-severe patients.