A thermal protecting device of a speaker is provided. The thermal protecting device includes an amplifier and a temperature detecting unit. An input end of the amplifier receives an audio reference signal, an output end of the amplifier provides an audio output signal to the speaker. The temperature detecting unit receives an audio input signal to provide the audio reference signal, and detects an operation temperature of the speaker to determine an amplitude of the audio reference signal. The amplitude is inversely proportional to the operation temperature.