An oscillating frequency drift detecting method, which comprises: receiving an oscillating signal with an oscillating frequency, wherein the oscillating signal is generated by a crystal oscillator; generating a self-mixing signal according to the oscillating signal; obtaining a self-mixing frequency of a maximum power of the self-mixing signal in a specific frequency range; and computing a frequency drift of the oscillating frequency, according to the self-mixing frequency of the maximum power, and the oscillating frequency.