Pipeline transportation is one of the most crucial methods for oil transportation, and in recent years, bimetallic pipelines have found increasing applications in the oil industry. However, buried pipeline operations over extended periods have brought about numerous issues that urgently need to be addressed. The magnetic gradient method utilizes the natural geomagnetic field as the excitation magnetic field source for pipelines. By employing a specially arranged array of magnetic flux gates, it enables non-excavation detection of corrosion defects in the active pipeline body, swiftly identifying the defect’s location. This paper focuses on the issue of corrosion leakage in buried bimetallic pipelines and validates the feasibility of weak magnetic detection technology based on the magnetic gradient method in the non-excavation detection of bimetallic pipelines. According to the simulation results analysis, the magnetic signal in the magnetic gradient detection method exhibits a peak-and-valley pattern at the defect location, consistent with the results obtained from the magnetic gradient method detection.