To achieve influence of underwater windmill's flow parameters and structure parameters-related changes with the hydrodynamic characteristics, an underwater windmill's additional contraction device is devised. According to the orthogonal experimental design, flow velocity, shrinking angle, locating range and outlet diameter are selected as four factors to carry out an 4 L-9 (3(4)) orthogonal experiment in this paper. The results of numerical simulation show that the influences of output torque and axial force on hydrodynamic performance of underwater windmill are flow velocity, locating range, outlet diameter and shrinking angle by turns. And the influence of flow velocity and locating range on output torque and axial force of underwater windmill is more evident, while the shrinking angle has small influence on it. As the output torque increases, the inhomogeneity of hydrofoils' surface pressure distribution is much more in evidence. No obvious flow separation phenomenon is occurred on hydrofoils' surface. It indicates that hydrofoil design is reasonable to some extent. In the upper edge of hydrofoil's pressure value are much lower, namely, this position may exist cavitations region. With the increase of the radius, the cavitation region moves to the trailing edge gradually. Mechanics analysis results also show that the assumption of rigidity hydrofoil is reasonable in the process of numerical simulation. Above research conclusions will provide significative reference for more hydrodynamic performance research of underwater windmill.