The invention provides a forming mechanism for an inner core paperboard of a filter and relates to the technical field of filter machining equipment. The forming mechanism is arranged on a machine frame. A thick carrier roller and a thin carrier roller are arranged on the machine frame. Rack plates are arranged on the thick carrier roller and the thin carrier roller correspondingly. The thick carrier roller and the thin carrier roller are arranged in parallel, and the rack plates are arranged in an engaged mode. A driving motor is fixed outside the machine frame, and the driving motor is connected to the thick carrier roller and the thin carrier roller through a rotary shaft. The forming mechanism for the inner core paperboard of the filter has the beneficial effects that the forming mechanism is novel in design and simple in structure, the carrier rollers with the rack plates are used for folding an inner core, and an original horizontal folding way is replaced, so that the effect is more remarkable, time and labor are saved, and folding is fast and convenient; and operation is easy, and the practicability is high.