Research purposes:With the rapid social and economic developments in China，there are more and more underwater tunnel projects，but its matching technical specifications and relative technical standards on the fire extinguishing system have not been formed a complete set of system． There is no clear specification on the fire extinguishing system for the underwater tunnel in the existing railway and highway codes，so these codes are unavailable for design of the fire extinguishing system for underwater tunnel． This paper analyzes the fire characteristics and the fire causes of the underwater tunnel，studies the current situation and problems in the fire extinguishing system for the existing railway and highway tunnels，and combined with the projects of the underwater railway and highway tunnels，gives out the key design parameters on the fire extinguishing system for the underwater highway and railway tunnels to provide the reference to the similar works．
Research conclusions:On consideration of the importance of the fire extinguishing system for the underwater railway tunnel，it is recommended to add the indoor tunnel fire hydrant system and the high － pressure water mist fire hydrant in rescue area and the different fire extinguishing systems should be used for the different long highway tunnels of less than 500m，from 500m to 1000m and from 1000m to 3000m．
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