Disclosed is a composite heat-dissipating system and its used fan guard which can impart a supercharging function to the heat-dissipating fans of the composite heat-dissipating system for efficient heat dissipation and reduces the noise generated when the heat-dissipating fans are operated. The fan guard includes a frame and a set of guard blades arranged inside and fixed onto an inner surface of the frame. The fan guard can be arranged upstream or downstream of the rotor blades of the heat-dissipating fans and assembled with the heat-dissipating fans in series or in parallel to supercharge the airflow out of the heat-dissipating fans.