Abstract: Three-core submarine cable lines are commonly used for offshore wind power transmission. In the relevant planning design and simulation analysis, the design of sequence parameters of three-core submarine cable lines is one of the foundations. Based on electromagnetic transients program (EMTP), a 220 kV submarine cable line with a section of 3×500 mm² are taken as an example, the sequence parameters of three-core submarine cable lines are calculated and analyzed. The radial resistance of semi-conductive polyethylene sheath is calculated, and the treatment method of metal sheath is discussed. The formulas for calculating the positive and zero sequence impedances per unit length, metal sheath and armor currents of three-core submarine cable lines are derived. The influences of magnetic permeability of steel wire armor on the current distribution and sequence parameters of three-core submarine cable lines are studied. In addition, the influences of optical fiber unit armor on sequence parameters of three-core submarine cable lines under different grounding conditions are also analyzed. The research shows that zero sequence resistance of three-core submarine cable lines is much greater than positive sequence resistance, and may even be greater than zero sequence reactance; when the equivalent relative magnetic permeability μ_r is within a certain range, the zero sequence reactance may be less than the positive sequence reactance. The simulation results are compared with measured results. The results show that reasonable calculation results can be obtained by selecting appropriate magnetic permeability of steel wire armor.