Abstract: An FM signal transmitter has a processing module, a connecting module and an FM transmitting module. The connecting module is electronically connected to the processing module and has a Universal Serial Bus (USB) unit that is selectively connected to a computer and receives audio data from the computer, wherein the USB unit has a memory storing a preset USB class. The FM transmitting module is electronically connected to and controlled by the processing module to broadcast an FM signal corresponding to the audio data under a specific frequency. The computer may perform an audio filtering method to successively rewrite the preset USB class from USB-HID, USB-AUDIO and USB-HID so that the computer can transmit audio data excluding system generated sounds to the FM signal transmitter.