The present invention relates to a trailer with a trailer brake control device therein. The trailer brake control device of the present invention comprises: a sway detecting sensor arranged in the trailer for detecting the sway of the trailer; a signal sensor for detecting a brake signal generated by a brake switch of a vehicle which tows the trailer; and a processor, electrically connected to the sway detecting sensor, the signal sensor and the brake, for controlling the brake based on an output of the sway detecting sensor and/or an output of the signal sensor. According to the present invention, the trailer brake control device can independently control the brakes and/or diagnose the malfunction of a brake controller arranged in the vehicle.