This review aimed to evaluate the impact of obesity on the onset, exacerbation, and mortality of COVID-19; and compare the effects of different degrees of obesity. PubMed, EMBASE, and Web of Science were searched to find articles published between December 1, 2019, and July 27, 2020. Only observational studies with specific obesity definition were included. Literature screening and data extraction were conducted simultaneously by two researchers. Random-effects model was used to merge the effect quantity. Sensitivity analysis, subgroup analysis and meta-regression analysis were used to deal with the heterogeneity among studies. Forty-one studies with 219543 subjects and 115635 COVID-19 patients were included. Subjects with obesity were more likely to have positive SARS-CoV-2 test results (OR = 1.50, 95% CI: 1.37-1.63, I2 = 69.2%); COVID-19 patients with obesity had a higher incidence of hospitalization (OR = 1.54, 95% CI: 1.33-1.78, I2 = 60.9%); Hospitalized COVID-19 patients with obesity had a higher incidence of ICU admission (OR = 1.48, 95% CI: 1.24-1.77, I2 = 67.5%), invasive mechanical ventilation (OR = 1.47, 95% CI: 1.31-1.65, I2 = 18.8%), and in-hospital mortality (OR = 1.14, 95% CI: 1.04-1.26, I2 = 74.4%). A higher degree of obesity also indicated a higher risk of almost all the above events. Region may be one of the causes of heterogeneity. Obesity could promote the occurrence of the whole course of COVID-19. A higher degree of obesity may predict a higher risk. Further basic and clinical therapeutic researches need to be strengthened. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.