A three-dimensional flow field of a small radial-flow turbine has been investigated by two methods of numerical simulation in this paper. The characteristic curves and primary parameters of different rotation speeds have been obtained. Through comparison and analysis, the result shows that the characteristic curves calculated by the quasi-three-dimensional method and by the three-dimensional method are basically the same. The characteristic curve can be obtained by using the quasi-three-dimensional method which needs shorter time than by using the three-dimensional method. By analyzing the characteristics of the flow field of the three dimensional numerical simulation, the initiative knowledge of the flow rule inside the radial-flow turbine is obtained, which shows that the flow losses exist, in different degrees, nearby the leading edge and the trailing edge of the blade at the design point. The numerical simulation can be used in the improvement and redesign of the turbine.