The thermal insulation of fire fighting system is very important for safe operation of highway tunnel in cold regions. In this paper, the calculation model of fire fighting watersupply pipeline insulation is established, and the volume of water released of trunk fire fighting watersupply pipeline and water releasing time of branch fire fighting watersupply pipeline are calculated by means of analysis based on heat transfer theory. Then the thermal insulation layer thickness is determined, and the temperatures of branch fire fighting watersupply pipeline without thermal insulation layer and that with a specific thickness of thermal insulation layer are numerically simulated by means of ANSYS software. The results show that: 1) The thermal insulating effect improves as the thickness of thermal insulation layer increases. The rational thermal insulation thickness is 0.06 m to 0.08 m. 2) The volume of water released is dominated by the trunk fire fighting watersupply pipeline and the water releasing time is governed by the branch fire fighting watersupply pipeline under a specific ambient temperature and a specific thermal insulation layer thickness. The principle of “water released frequently by branch fire fighting watersupply pipeline and volume of water released controlled by trunk fire fighting watersupply pipeline” has to be followed. 3) The numerical results show that the requirement of fire fighting watersupply pipeline can be met when the thermal insulation layer thickness reaches 0.08 m and interval time among water releasing reaches 8 h. The paper can provide reference for thermal insulation design of fire fighting watersupply pipeline of tunnels in cold regions.