Abstract: To reveal the mechanism of vertical flame spread in flame-retardant cables, combining experiments with numerical simulations of fire dynamics simulator (FDS) based on Tests on Electric and Optical Fibre Cables under fire conditions—Part 11: Test for Vertical Flame Propagtion for a Single Insulated Wire or Cable—Apparatus (GB/T 18380.11/12—2022), combustion behavior of WDZA-BYJ 450/750 V cables under jet flame was systematically analyzed. Experiments with different conductor sizes (cross-sectional area of 2.5, 4.0, 6.0 mm²) and flame exposure times (2~180 s), together with numerical simulations, a four-stage model of flame-retardant cable combustion was validated: Stage I (initial pyrolysis), Stage II (self-sustaining flame driving), Stage III (self-sustaining flame attenuation), and Stage IV (conductor-dominated heat transfer). Both experimental and simulation results consistently showed that increasing conductor size could significantly suppress combustion intensity, with heat release rate of the 6.0 mm² cable being 37% lower than that of the 2.5 mm² cable, and the mechanism was attributed to physical blocking effect of the larger conductors, which could weaken heat transfer to the rear side. Besides, copper/aluminum conductors had a weak influence on flame spread of thin cables (cross-sectional area ≤2.5 mm²), while conductor size could dominate flame spread process by altering heat feedback path. The study could clarify that coupling effect of "flame—insulation layer pyrolysis—conductor heat transfer" was the main control mechanism for vertical flame spread, providing a theoretical and technical basis for the design of high-safety cables and fire prevention.