In the automatic fire alarm and linkage system, a dedicated fire telephone system is an integral part. In a large building, fire telephone extensions are installed in the power distribution room, the fire fan control room, the fire pump room and the gas fire extinguishing operation devices, so that one can communicate directly with the fire control room by picking up the telephone; At the same time, fire telephone jacks are also installed next to the manual fire alarm buttons evenly distributed in the building, so that one can talk to the fire control room by inserting a portable telephone extension in case of emergency. According to the communication principle of fire telephone host and terminal equipment, fire telephone systems can be divided into two types, bus and multi-line, and their different modes of operation have an impact on the length limits of fire telephone lines. This paper examines the limiting factors for the length of telephone lines in dedicated fire telephone systems and gives the limit lengths (maximum lengths) of various commonly used telephone lines for application in fire telephone systems through calculation. According to the engineering calculation table given by the author, the main factors affecting the call quality of the fire-fighting special telephone system are the interline insulation resistance and interline capacitance.