Monitoring the oil scale deposition thickness of pipelines is beneficial to ensuring the efficient and safe operation of pipelines. In this paper, an improved gamma-ray transmission method is proposed to reconstruct the two-dimensional (2D) oil scale profile of pipelines. The method combines the gamma-ray transmission method and scanning technology to measure the deposition thickness of the oil scale and rotates the gamma-ray scanning direction to different angles, after completing a transmission scanning process, to achieve the full-angle measurement of the oil scale deposition thickness. Based on this method, a set of oil scale profile detection devices is designed and the detection process is simulated by the Geant4 toolkit. In this system model, the pipelines with and without oil scale are scanned, respectively, by using the single-energy gamma-ray beam to analyze the relative transmittance of gamma rays at the energy of 0.662 MeV. The results show that the approach is efficient for detecting the deposition thickness of oil scale in oil pipelines and is accurate for the 2D oil scale profile reconstruction of a pipeline. The maximum deviation is about 0.59 cm, and the relative error is less than 5%.