An ultra-low power transmitter applied in multi-channel frequency shift keying (FSK) communication is provided. The transmitter includes a fixed-frequency generation device, a low-frequency frequency synthesizer, and an injection locking device. The fixed-frequency generation device provides a reference frequency to the low-frequency frequency synthesizer. The frequency synthesizer divides the reference frequency with corresponding divisors for generating a plurality of divided frequency signals. Then, the divided frequency signals are injected into the injection locking device. The injection locking device will lock at the average frequency of previously mentioned divided frequencies. Wherein, the injection locking device filters the high frequency noise, which is produced by the frequency synthesizer, at the time of the injection locking. The ultra-low power transmitter obtains a high-frequency transmitted signal by using the frequency-locked signal.