A height adjusting apparatus, for adjusting the height of a projector, includes a support leg, an air-pressure cylinder connected to the projector, and a switch. The air-pressure cylinder and the support leg together form a sealed chamber. When the pressure of the air inside the chamber is regulated, the height of the air-pressure cylinder and the height of the projector change. The switch is mounted on the air-pressure cylinder to create a gap to allow air to exit or enter the chamber when the switch is open.