A rack-mount server system includes a cabinet, a number of server units mounted in the cabinet, and a number of fan modules. Each fan module cools one or a group of server units. Each fan module includes a mounting bracket fixed to the cabinet, a number of fans attached to the mounting bracket, an air flap, and a control box to control the power supply to the fans. The mounting bracket defines an air passage dedicated to each fan, through which air from the fan is guided to the one or a group of server units. Each mounting bracket defines a cutout, through which the air passages of two neighboring fans communicate with each other. The air flap may be slid closed or slid open to redirect or gather a cooling airflow as required, depending on the distribution of server units within the cabinet.