AN MP3 broadcasting device, which can be used via plugging in a cigarette-lighter socket inside a car, has a cigarette-lighter plug, a frequency modulation (FM) radio module, a transmission module, a playing module, a control module and a frequency-setting module. The cigarette-lighter plug is used to obtain a first electric power from the cigarette-lighter socket and convert the first electric power into a second electric power suitable for the MP3 broadcasting device. The transmission module is used to connect with an external electronic device capable of outputting music signals to obtain the music signals. The playing module is used to process and play the received music signals, and then transmits the playing music via the FM radio module. Therefore, a user can use a radio inside the car to listen to music transmitted by the MP3 broadcasting device.