Abstract: A low frequency magnetic flux leakage detection technique was developed for the detection of the outer wall of the ferromagnetic pipeline with heat radiating fins. According to the structural characteristics of the pipeline, a low frequency magnetic flux leakage sensor was developed. By means of numerical simulation and experiment, the detection ability of the sensor was studied in different depths and widths of the defect in the coated aluminum pipe. Results show that the amplitude and the harmonic distortion rate of the detected signal can be used for the quantitative detection of defect, and when the pipe is coated with aluminum, the intensity of signal detection of defects increases. Therefore, the low frequency magnetic flux leakage sensor has a better effect on the outer wall damage detection of the coated pipeline in the actual working conditions.