Until now, most of the hydraulic hand tools for underwater application still use mineral oil as the system fluid medium. Due to the incompatibility with marine environment, these systems have highlighted a series of disadvantages. Therefore using seawater hydraulic tools has become a tendency in ocean engineering. In this paper, the design on a novel seawater hydraulic wirerope cutter for divers was introduced in detail based on its original prototype. A test rig with computer data acquisition system using Matlab/xPC was developed for testing the tool's performances. A group of wireropes with different diameters were used as specimen and they were cut off smoothly by the new developed tool in all the tests. The maximum pressures and necessary cutting forces for cutting off wirerope specimen were obtained by experiments. The results show that the cutting forces have linear relationship with the cross-sectional areas of the wireropes. It's verified by experiments that the tool is successfully designed and it has been applied in underwater application.