We proposed the new type turbine, double rotor turbine, for OWC to improve the performances of starting characteristics and efficiency. The splitter plate of the double rotor turbine is the most important part that divides the flow into Wells or Impulse rotor. It is discussed the influence of the splitter duct length to the performances in numerically. A torque is gradually decreased with the splitter duct shorter, but the variation is small. It is kept the large torque to start up the turbine. The maximum efficiency is a little larger than Impulse turbine, but lower than Wells turbine. The efficiency becomes maximum at 0.5 length rate. The work rate of Wells rotor is large at small splitter duct, because of the large flow rate to the Wells rotor. The variation of the efficiency with the flow rate is almost same to the duct length. The starting characteristics is improved from the wells turbine at all duct length. At long splitter duct, it is remarkable the flow into the wells rotor wraps around the edge of splitter duct. The flow rate of wells rotor is smaller than impulse turbine.