Ocean thermal energy conversion (OTEC) is a method of power generation which harnesses the deference of temperature between warm sea water at ocean surface and abyssal cold sea water. Recent studies on OTEC plants are being carried out through a pilot plant which substitutes a heating and cooling source system for a part of warm and cold sea water. However, the efficiency of the conventional water supply system such as one tank system is not good enough to use as a OTEC pilot plant, because it takes much time to supply the appropriate water. Further it is impossible to make the desired water temperature and flow-rate at a constant level during the experiment. In this paper, a new temperature flow-rate water supply system with closed cycle is proposed for solving the existing problems. Proposed system is rather convinient to use as an OTEC pilot plant due to two main advantages. First is the ability of quickly supplying the objective water with an appropriate temperature and flow-rate. And the other is the possibility of conducting an experiment for a long time by adopting the closed cycle water supply system. In order to construct the procedure for improving the controllability of this system theoreticaly, controllable range of the pumps was also investigated and evaluated.