Because of the limit of the structure and the environment of the power cable, it is very difficult to raise the capacity of the cable. In order to solve such problems, it is necessary to use the special measures to increase the capacity of cable. Although there are many measures to expand the capacity, using the water cooling system is the comparably cheap, effective technique. This paper analyses the water cooling system of cable in the tunnel, the result of analysis indicates that the cooling system will raise the capacity notably and it has the practical value in the project. On the other hand, the paper discusses not only the composition and space of the cooling system, but also the running and maintenance of the system. All results of this paper can be used as the technical reserves for the practical application.