A detecting device for detecting a rotation speed of the color wheel includes: a first processing unit, used to compare an external ambient temperature with a preset temperature, and outputs a logic level when the external ambient temperature is greater than the preset temperature; and a light-emitting unit, including a light-emitting body for emitting measurement light, a temperature compensation circuit, wherein the temperature compensation circuit adjusts, according to the logic level and the external ambient temperature, the resistance of a circuit in which the light-emitting body is located, causing a driving current of the light-emitting body to be within a preset current range, a reflection unit disposed on a moving color wheel and used to reflect measurement light, and a measurement and control unit used to receive the measurement light emitted periodically by the reflection unit, and to calculate the number of times that the measurement light enters.