Abstract: COVID-19 is a global epidemic disease. This paper holds that diabetes and COVID-19 have a common endogenous basis for spleen deficiency. Besides， dampness is not only a pathological product in the course of disease development but also an important pathogenic factor. The clinical manifestations of diabetes patients are spleen deficiency and dampness excess， defensive qi lack of source， insecurity of defensive qi， susceptible to pathogenic factors， and then catch diseases， abnormal fluids transportation results in dampness invading lung. When diabetes patients caught COVID-19， the phlegm-dampness will attack the lung upward， which would aggravate symptoms such as cough， phlegm， and puff and increase the risk of poor prognosis and death. On this basis， spleen deficiency and dampness excess is the crucial factor for diabetes patients to catch COVID-19 and exacerbations.