Abstract: In order to study an eco-friendly and simple waterwheel device with a micro water head, a three-dimensional model of the waterwheel in the river was established. The volume of fluid（VOF） model was used, and the pressure-velocity coupling was solved in the PISO format. The width-to-dia-meter ratio of different impellers was calculated. Unsteady flow analysis was performed on the waterwheels. The results show that the chest shot waterwheel with an impeller width-to-diameter ratio of 3.93 has the widest efficiency zone and the highest optimal efficiency. The increase of the width-to-diameter ratio of the impeller increases the blocking effect of the flow channel, and the water level difference between the upstream and downstream increases, and the obvious vortex phenomenon appears.Torques of waterwheel blades with different width-to-diameter ratios show the same periodic change, when the waterwheel is running in the dry season, there will be a backflow phenomenon between the two blades immersed in the water during the movement of the blades, so that the torque has a small range of extreme values. The pressure between the two blades that are not immersed in water increases as the width-to-diameter ratio of the impeller increases. The front pressure of the waterwheel blades of five width-to-diameter ratios reaches the maximum at the tip of the blade, and the waterwheel with an impeller width-to-diameter ratio of 3.93 has the largest pressure difference between the front and back sides of the waterwheel blades, which improves the blade′s performance. The results play a guiding role in the design of the impeller structure of the chest shot waterwheel.