The icing of transmission lines has seriously affected the safe operation of the power grid. Existing transmission line icing models mostly ignore the axial icing difference and consider the key icing parameters as time-invariant (single-step) conditions, while there are few reports on three-dimensional ice accretion model with time-varying (multi-step) parameters. Based on lubrication theory and line icing mechanism, this paper proposes a transmission line icing model considering the influence of time-varying icing parameters. By adopting the ANSYS-Fluent ICING module, the icing calculation for time-varying parameters was carried out on the three-dimensional line model. The validity of the calculation method was verified by using actual transmission line icing test data, and the calculation results were in good agreement with the test results. On this basis, the single-step and multi-step ice accretion calculation methods were compared. The influence of the transmission line inclination angle and line diameter on the shape and mass of ice accretion was analyzed. Results show that the accuracy of multi-step icing calculation method was about 8% higher than the single-step icing calculation method. Under the condition of dry ice coating, with the increase in the inclination angle of the transmission line, the shape and mass of ice accretion on the transmission line had no obvious change. Under wet icing conditions, the inclination angle of the transmission line had a significant effect on the ice accretion. As the inclination angle increased from 0° to 60°, the ice coverage area decreased and the shape of ice accretion gradually became smoother, but the ice accretion mass decreased by about 21%. The calculation results of ice accretion on transmission lines with large and small diameters were compared, and the amount of ice accretion on lines with large diameters was significantly higher than that on lines with small diameters.