A control method for cooling a projection display, wherein a pressure sensor automatically detects the elevation above sea level of a projection display and then transmits a signal to the control unit for controlling the cooling unit to operate in a corresponding cooling mode, thereby the cooling efficiency varies with the elevation above sea level and then the cooling unit can operate in a corresponding rotating speed in order to effectively cool the whole projection display and increase the flexibility in using the projection display, and further extend the lifetime of each component inside the projection display.