A backflow prevention device includes a plurality of flaps that are connected pivotally to a casing of a fan. When a blade unit of the fan rotates to generate an airflow entering the casing via an airflow inlet and exiting the casing via an airflow outlet, the flaps are driven to pivot away from the blade unit to open positions, where the flaps serve as stator blades for the fan. When rotation of the blade unit is stopped, the flaps are driven by a reverse flow directed toward the airflow outlet and the blade unit to pivot toward the blade unit to closed positions, where the flaps close the airflow outlet to prevent air from entering the casing via the airflow outlet.