Aiming at the high-efficiency OTEC application, a 30 kW radial-inflow turbine using ammonia was constructed via the one-dimensional design. In addition, a 3D CFD simulation was carried out to research the turbine performance. Furthermore, the effects of blade tip clearance on the flow characteristic in turbine were elucidated and the turbine performance under the off-design conditions was examined. It is indicated that the performance of the designed turbine is satisfactory with a theoretical isentropic efficiency of 85.2% under the design condition. The leakage flow encounters the scraping flow after leaving the tip clearance, forming the vortexes in the cross-section of the flow passage, which decreases the turbine efficiency, and the best ratio of blade tip clearance to inlet blade height is suggested to be 0.04–0.06. When the rotational speeds deviated from the design value, the vortexes appear in the flow passage of the turbine, resulting in non-negligible impact loss of blade and the consequent efficiency reduction. At the design rotational speed, the turbine shows a good adaptability to the inlet temperature variations with the inlet temperature varying from T0 = 296K to T0 = 304K. When the inlet pressure increases, the rotational speed corresponding to the maximum efficiency is also increased.