Variation characteristics of water level and Manning roughness coefficients of an open channel with submerged vegetation are studied with flume experiments under different plant spacing and different flow conditions. An empirical formula for the Manning roughness coefficient of the open channel with submerged vegetation is obtained. The results show that the backwater and surface slope of the open channel increase with the decrease of plant spacing under the conditions of the same discharge and water depth. Influence of the submerged vegetation on the water level and surface slope is relatively larger when plant leaves touch or cross each other. Under the conditions of the same water depth and plant spacing， surface slope presents linear increases with the increase of flow. The influence of flow velocity on roughness of the open channel with the submerged vegetation is little under the same water depth.