introducing a to-be-treated fluid into a pipeline to flow to form a cycle; wherein the fluid in the cycle is driven by at least one driving device connected to the pipeline;

discharging a treated fluid from the cycle, and retaining a fluid margin in the cycle;

a total amount or total velocity of the to-be-treated fluid introduced into the pipeline is equal to that of the treated fluid discharged from the cycle, so that a total amount of fluid in the pipeline is in a state of dynamical equilibrium;

wherein the fluid treatment method is carried out by at least one cycle, the at least one cycle further comprises a catalytic unit, the catalytic unit catalyzes the fluid in the cycle to change a structure of a target substance in the fluid and generate a target product; or the catalytic unit changes a concentration of the target substance; the catalytic unit is a cyclic catalytic unit participating in the cycle along with the fluid margin; the catalytic unit comprises a biocatalytic unit and a non-biocatalytic unit;

the cycle further comprises an interception module arranged in the pipeline, the interception module is a tangential flow filtration (TFF) module, the interception module has an interception component to intercept the cyclic catalytic unit to prevent it from leaving the cycle; the driving device controls the fluid in the pipeline to flow at a preset flow rate so that the catalytic unit flows to the pipeline through the interception module; the interception component divides the interception module into a first side and a second side, and the cycle comprises at least one of the following:

a first-type metabolic cycle, wherein an entrance and an exit of the pipeline are connected to the first side of the interception module, and the first-type metabolic cycle has at least one inlet connected to the pipeline and at least one outlet connected to the second side of the interception module; and

a second-type metabolic cycle, wherein an entrance and an exit of the pipeline are connected to the second side of the interception module, and the second-type metabolic cycle has at least one inlet connected to the first side and at least one outlet connected to the first side of the interception module.