In order to improve the operating efficiency of a suspending type axial-flow tidal current turbine, which can be self-adaptive according to the direction of ocean current, a three-section duct with outer envelope has been installed around the turbine. The hydrodynamic performances of this duct have been analyzed with numerical simulation method and the results have been verified by model tests. It has been found that the duct with fold line shape, where the attacked angle of constringent section and proliferation section is 15°, has excellent hydrodynamic performances and the power output of turbine with duct is approximately 30％ more than that without duct under the same condition. Besides, the similar flow field data have been obtained from model tests. That is, the form resistance of constringent and dirergent sections is relatively small, so the enegy conversion effect is obvious for steady flow velocity in straight-line section. Therefore, the duct with fold line shape best matches this turbine scheme.