Power take-off is an indispensable link in wave energy utilization, and its efficiency should be comprehensively investigated at both full load and part load for the effective conversion in variable wave conditions. However, up to now, the research about this issue is still scarce because of its complexity and difficulty. To overcome this obstacle, this paper studies the overall conversion efficiency of the constant-pressure hydraulic power take-off (CPHPTO) of a floating-pendulum wave energy converter, at a wide range of the system pressure, system flow-rate and shaft speed, via the efficiency test and fitting formulas. Furthermore, the integrated characteristic curves, which consist of the characteristic curves and operating curves, are proposed to deal with the four-dimensional data obtained from the experiment or fitting. The results show that the integrated characteristic curves of the CPHPTO are available and play an important role in the optimal design and efficient operation. The stable operating region can also be defined when plotting the operating curves. In general, the integrated characteristic curves are suitable for the preliminary design and further optimization of the similar CPHPTOs for different kinds of oscillating-body wave energy converters.