A fan-driven heat dissipating device with an enhanced air blowing efficiency is provided, which is designed for use in conjunction with a casing of an electronic system, such as a server, for providing the server with a fan-driven heat-dissipating function, and which is characterized by the provision of a special casing structure that allows the maximum allowable blade length of the fan unit to be increased as compared to the prior art, allowing the fan unit to provide an enhanced air blowing efficiency so that it can more efficiently blow away the heat produced by the server during operation.