Studies on humoral responses to infections mediated by IgA are receiving increasing attention. It was proposed that secretory IgA might be a therapeutic strategy against SARS-CoV-2. However, there is no experimental data on anti-SARS-CoV-2 efficacy by IgA in serum. Here, we purified NP- and RBD-specific antibodies directly from convalescent patients’ serum pool, and determined the absolute mass concentration of all types of serum antibodies in COVID-19 patients. RBD-specific antibodies are able to block ACE2 binding to RBD and neutralize live SARS-CoV-2. RBD-IgA exhibited an impressive neutralizing potency, with an ND50 value of 2.42 ± 0.89 μg/mL, which is 10-fold better than that of RBD-IgG. This study provides direct evidence for the potent antiviral activityof serum-derived RBD-IgA. Furthermore, our results show that the serum pool from COVID-19 convalescent is more potent than individual purified RBD antibodies in terms of antiviral activities, suggesting the presence of other antibody blocking sites on the virus and additive effect of different antibodies, which offering a theoretic basis for convalescent serum therapy and combination antibody treatment strategy.