A chaotic fiber fence system based on a semiconductor fiber ring laser is proposed,in which the laser outputs are framing polarization chaotic light and a part of the fiber ring is taken as sensing fiber. Wherever the intrusion acts in the sensing fiber,it will disturb the distribution state of the birefringence in the fiber,which influences the initial state of the laser. Output waveform changes immediately because chaos is sensitive to its initial value. By a cross-correlation detection of the waveforms in adjacent two frames,the intrusion can be detected. The position of the ring can be converted to the time of the frame with the framing characteristic of the chaotic waveform. According to the relation between the intrusion position and cross-correlation peak value,the intrusion position is located. This method is real time and with stronger anti-interference capacity and the detection results are immune from external environment fluctuations. The experiment results illustrated that the system has a location error of about 45 m,the relative error of 5%,and a good repetitiveness with a 3 km long sensing fiber.