The utility model discloses a catalyst on-line addition device which can add on line, is simple to operate and convenient to manufacture, is low in investment cost, and is unnecessary to be daily maintained and detected. The catalyst on-line addition device comprises a catalyst injector, an inlet pipeline, an outlet pipeline, a bypass, an inlet pipeline valve, an outlet pipeline valve and a bypass valve, wherein the catalyst injector comprises the inlet pipeline, the outlet pipeline and the bypass and is communicated with the inlet pipeline and the outlet pipeline, and the inlet pipeline and the outlet pipeline are communicated with the bypass. In the catalyst on-line addition device, a filtering element is arranged at a catalyst addition port so as to prevent impurity in a catalyst solution from entering fluid. The device can be directly installed and improved based on the existing pipeline, is unnecessary to be daily maintained and detected, and adopts a manual addition mode, and a catalyst can be arbitrarily added at any time according to a production process.