The present invention discloses a fan counter-flow-preventive device which includes a panel and a plurality of flow-stopping components; a plurality of through holes are set on the panel; each flow-stopping component includes a flow-stopping section and a flow guiding section which connected with the flow-stopping section; each flow-stopping component is connected with a corresponding through hole on the panel through the flow guiding section on it; the flow-stopping section also includes a plurality of petal shaped objects that can open and close; a enclosed convex body is formed when the plurality of petal shaped objects closed; and the flow guiding section is hollow. The fan counter-flow-preventive device utilizes pressure difference of two sides of the flow-stopping section to open or close automatically without gravity effect, hence direction factor need not be considered when mounting, and the whole fan counter-flow-preventive device has simple lay out and low cost.