Backgrounds:The coronavirus disease 2019 (COVID-19) pandemic has killed more than 1.5 million and infected more than 65.8 million people in the whole world as of July 12, 2020. The scientific research data show that severe acute respiratory syndrome coronavirus 2 is etiologic agent of COVID-19. The virus invasion can damage alveolus capillaries. The wounds may cause the hemorrhage, coagulation, clots, and (or) thrombosis. The thrombi can block blood circulation completely or partially. Methods and Results: Based on our estimation with hemodynamics, the blood flow rate decreases about 40% or 34% when the diameter or height of the blood vessels decreases 10%. The complete or partial obstructions also slow down transportations of O 2 , CO 2 (conveying in red blood cells or plasma) and all nutrients in blood flows as well as increase risks of turbulence that can produce more thrombi, called "a vicious cycle" in hemodynamics. Therefore, fatal risks of ischemia or hemorrhage strokes as well as anemia increase. The viscosity of water is only 1/4-1/3 of that of human blood; drinking (boiled) warm water can decrease the viscosity as well as dissolve the blood clots to increase the blood flow and transportations of all nutrients without side effects. In addition, when people are (seriously) infected by COVID-19, they usually have fever, pulmonary hemorrhage or edema, and do not have much appetite to eat. Therefore, based on hemodynamics, we suggest drinking (boiled) warm water and complete nutrition fluids, for easy digestion, energy supplementation, and primary and secondary preventions of blood clots (or thrombi) in clinic. Conclusions: The complete nutrition fluids can provide all nutrients to maintain human bio-energy for daily lives and increase strength of immune system. Drinking warm (boiled) water can directly compensate the loss of water and the heat, keep the blood volume and reduce the blood viscosity without side effects to prevent or cure blood clots or thrombosis caused by COVID-19. Therefore, it is very important to drink (boiled) warm water and complete nutrition fluids to maintain transportations of all nutrients in blood flowing for primary and secondary preventions of COVID-19 in clinic.