A fan fixing device for fixing fan units includes two parallel first sidewalls, two parallel second sidewalls, and a number of fixing members. Each first sidewall defines a number of vent ports and includes a vertical plate, a lower plate, and an L-shaped engagement member. The lower plate and the engagement member are positioned at opposite sides of the vertical plate. The second sidewalls are connected between the first sidewalls. The second sidewalls and the first sidewalls cooperatively form a receiving room. The fixing members engage with between the engagement members of the first sidewalls, the fixing member is configured for securing the fan units to the two first sidewalls in the receiving room.