Abstract: The incipient motion of fine sediment is tested in a pressured rectangular pipe with different water head. The results show that the water pressure greatly influences the incipient motion of fine sediment in that the incipient sectional mean velocity and the incipient shear stress increase with water pressure. The flow in rectangular pipe can be converted to the flow in ordinary open channel according to the characteristics of flow distribution and water pressure in rectangular pipe. The converted vertical average velocity in open channel is basically the same as incipient sectional mean velocity in rectangular pipe. The results have verified the feasibility and representativeness of the study on incipient motion of fine sediment in rectangular pipe. The change of Manning roughness coefficient under different water head has not been considered in the conversion process of incipient velocity of fine sediment from rectangular pipe to open channel, so the laws in this area need to be further researched.