A line-based fiber optic sensor, modified from the loop-based configuration of the Sagnac interferometer using a Faraday rotator mirror (FRM), is easy to set the sensing fiber (SF) in the field application, especially in long-distance perimeter security. The sensor we proposed is composed of two line-based interferometers, which are multiplexed with super luminescent diode (SLD), interference unit (IU) and SF respectively through wavelength division multiplexer (WDM) to obtain different wavelength disturbance signals generated by the same disturbance. Two signals with fixed time delay, determined by the position of the disturbance and extracted by signal correlation method, are obtained. Then the position of disturbance can be calculated by it. Experiments show that low-frequency noise has a great impact on the positioning results, and even leads to the failure of positioning. In this work, the influence of disturbance frequency and sampling rate on the accuracy of calculation results is analyzed, and two new processing methods is proposed. Firstly, we use bandpass filtering to eliminate the effects of these factors. Disturbances at different locations require different filtering ranges. We propose a method to determine the filter range by null-frequency, which can achieve higher positioning accuracy. Secondly, the sampling rate of 0.5 MHz is raised to 2.5 MHz by using spline interpolation method. The combination of these two methods can achieve higher accuracy and better reliability of positioning results. Experiments results show that the positioning accuracy of this method can be improved from 200 m to 40 m with the total length of SF is 40 km.