In this paper, we measure a work efficiency of the rock crushing operation of the underwater robot hydraulic system and presents effective parameters for the maximum work efficiency. Because of the power resource is not enough to work in underwater environment for a long time, it is important to know the work efficiency of the robot system. First, we select several parameters to calculate power about the rock crushing work, and grasp the characteristics of each parameter. Second, we compute a maximum power and actual work power of the hydraulic tool in order to calculate the work efficiency by analyzing the test data of the rock crushing. Finally, according to the compressive strength of the rock, we show what are parameters affecting to maximize the work efficiency. In the rock crushing test results of the specimens, the parameters of the differential pressure between input and output hydraulic ports of the rock crusher under the low compressive strength and the rotational speed of the tool under the high compressive strength has most influence on the efficiency of the work.