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Influenza viruses continue to cause epidemics worldwide every year. However, due to the lack of an effective assessment for the severity of influenza epidemics, it was extremely difficult to take preventative measures. Data were extracted from infectious diseases reports from 2011–2018. Joinpoint regression model and susceptible-exposed-infectious-recovered model were built to understand the characteristics and processes of the epidemic. The reported incidence of influenza was 1,913,698 from January 2011 to February 2018, with an average-yearly-reported-incidence-rate of 19.21 per 100,000. However, there had been a substantial nationwide epidemic of influenza after September 2017, when the average yearly reported incidence rate was 87.29 per 100,000 and an annual percentage change of 48.1%. The hemagglutinin genes of most influenza A (H1N1 and H3N2) viruses from the period of the epidemic had lower homology to those before August 2017. All the hemagglutinin of the recommended A (H3N2, H1N1) and B (Victoria) viruses for vaccines 2017/2018 had low matches with the epidemic viruses. The basic reproduction number was 1.53. The vaccination benefit was linearly related to vaccination coverage, while the quarantine measure had only significantly benefited when over 60% of the quarantined population. The most severe epidemic of influenza in China since 2011 occurred during the period from September 2017 to February 2018. Compared to quarantine, influenza vaccination is more effective way to prevent influenza, and strategies to increase vaccination coverage should be taken for the prevention of severe epidemics of influenza.