The movement state of contaminated oil in the pipeline is of great significance to the safe operation of oil-using equipment. The dynamic motion characteristics of the oil can be characterized by signals. However, the pressure signal of the oil is time-varying and complex, hence it is a typically non-stationary nonlinear signal. Therefore, the traditional linear analysis method used for the analysis of the oil signal is not suitable. For this reason, the Hilbert-Huang Transform (HHT) method is used to process and analyze the differential pressure signals of oils with different degrees of pollution, to obtain the characteristic frequencies of oil pressure signals, to explore the intrinsic connection of the characteristic frequencies and oils with different degrees of pollution, and to reveal the dynamic movement characteristics of oil in the pipeline. The results show that the characteristic frequencies corresponding to the five groups of oil samples with a pollution degree of 17/12, 18/12, 19/13, 19/13, 20/16 (ISO4406 standard) are 20. 29 Hz, 10. 22 Hz, 6. 94 Hz, 17. 01 Hz, and 6. 81 Hz, respectively, Each Intrinsic Mode Function (IMF) component of the oil signal has obvious frequency modulation characteristics, As the pollution degree increases, the oil frequency of the IMF2-4 component mainly shifts toward the middle of the interval, and the oil frequency of the IMF5-7 component mainly shifts toward the direction of 5. 00 Hz, 3. 00 Hz, and 1. 60 Hz respectively.