The fiber Bragg grating is sensitive to both the temperature and the strain on sensors, while the trends of their wavelength drifts differ to each other a lot. A special fiber fence monitoring system based on the fiber Bragg grating sensing network is investigated, which can realize the perimeter intrusion detection and the area fire protection simultaneously in a single sensing system configuration. A signal processing method is proposed which can intelligently tell if there is any threat and which kind of event it is according to the extracted signal characteristics in the time and frequency domains. Thus a normal perimeter security system possesses a second function of fire prediction, without any additional temperature sensing cables or other fire alarm systems in parallel. The results show the effectiveness of the method.