The spiral water turbine（screw-type turbine, spiral water mill）has a runner whose shape is the same as that of Archimedes pump. As this turbine can run on the very low head situation, and it can be very easily set to open water channel, it is suitable for the very small hydraulic power generating. In this study, the internal flow and its relation to the generated power are analyzed both by model experiment and by CFD. The CFD result agrees very well with that of model experiment, assuming the torque at free runner status. A formula to estimate the performance of this turbine is proposed, which is based on the momentum equation of fluid.