An adjustable air filter for a computer, includes a bracket, a plurality of frames with air filter units set therein rotatably mounted to the bracket, a first transmission bar connected with each of the frames, a second transmission bar rotatably mounted to the bracket and connected with the first transmission bar, and a driver fixed to the bracket with a shaft retractably extending therefrom. The first transmission bar drives the frames to rotate when the first transmission bar is moved. The second transmission bar drives the first transmission bar to move when the second transmission bar is rotated. The shaft of the driver is connected with the second transmission bar. The driver drives the shaft to extend out or draw back according to the temperature in the computer to drive the second transmission bar to rotate.