The COVID-19 pandemic broke out in Wuhan, China, and declared an international public health emergency by the World Health Organization in 2019. It mainly manifests as symptoms of respiratory infections, and severe cases can cause pneumonia and death. The Diamond Princess cruise ship outbroke cluster infection outside China during the early pandemic. The incident occurred on February 1, 2020, and an 80-year-old Hong Kong man was diagnosed with COVID-19. The cruise docked in Yokohama, Japan, for 14 days on-board quarantine; however, cluster infection outbroke rapidly. The results show that after 14 days of quarantine, 634 (17.1%) cases were diagnosed with a total of 3,711 population, and 328 (51.7%) cases were asymptomatic. As of April 24, 2020, 712 cases have been diagnosed and 14 deaths have occurred. A cumulative mortality rate reaches 1.96%. Using a nonlinear least-squares curve fitting with Microsoft Excel Solver, we obtain the parameters of the SIR mathematical model of infectious disease and the reproduction number (R0) of the COVID-19 outbreak is 2.37±0.26. Without an emergency evacuation plan, the total infection rate will reach 88.47%. These data show “only one” COVID-19 case could still outbreak cluster infection on large cruise ships. The possible causes and countermeasures are discussed.