In this paper, a fire model of a high-voltage cable tunnel is constructed by numerical simulation method, and the diffusion law properties of hydrogen chloride (Hcl), the characteristic gas of the fire, are analyzed under the conditions of different ignition source locations and ignition source power. The results show that the temperature in the high-voltage cable tunnel rises slowly at the beginning of the fire (25 s ago), then increases rapidly, and stabilises after 100 s. The closer the fire source is to the top of the tunnel, the more significant the temperature difference in the tunnel and the lower the diffusion rate of Hcl gas. The higher the power of the fire source, the faster the temperature rises and the more uniform the Hcl concentration distribution. When the power of the fire source reaches 6 MW, the distribution of Hcl gas in the tunnel is uniform. These findings provide an important reference for the safety design and fire protection of high-voltage cable tunnels.