Abstract: To solve the problem of fire alarm in utility tunnel, the installation distance of fire detector was arranged reasonably. In this paper, a mathematical model of conversion coefficient of heat energy from fire source was presented. Through the establishment of a three-dimensional utility tunnel, a simulation study was conducted by using FDS software. The influence of different width and height of utility tunnel was analyzed on the temperature. In addition, the correctness of the mathematical model was verified by comparing the mathematical model and the simulation results with previous research results. Results show that the farther away from the fire source, the longer the maximum temperature of hot smoke at the top of the utility tunnel. The thickness of the hot air layer and the smoke layer are affected by the width and height of the utility tunnel. The conversion coefficient of the heat energy of the fire source is positively correlated with the equivalent diameter of the utility tunnel. When the width and height of the section of the utility tunnel varies from 1.40 m to 2.60 m, the installation distance of fire detector is suggested to be 5.00 m, and the location of fire source can be determined within 30 s.