A fluid treatment method, cycle treatment device, system and medical device are provided, wherein a cycle is formed by allowing a fluid to flow in a pipeline, and the cycle includes a treatment unit to treat the fluid to selectively change structures or concentrations of molecules or combinations thereof in the fluid, thereby avoiding loss of beneficial components; the fluid is treated by at least one cycle, and in any cycle, dynamic equilibrium of the total amount of the fluid in the cycle can be maintained through controlling the rate of adding the to-be-treated fluid into the cycle and the rate of the treated fluid leaving the cycle, so that the cycle is sustainable, and therefore the duration of the fluid treatment method is adjustable and can be determined based on a preset treatment target.