The pebble is often used in river model in the past. In view of the ecological environment protection of pebble prohibition and the characteristics of easy access and high roughness of gravel，we study the resistance of gravel. We obtain the representative element weight of homogeneous gravel particles and the average constant volume particle size of multiple types of gravel by statistical analysis method，carry out the experiment of the steady flow in open channel with the dense bed gravel and cement slurry wall，apply the river bed and bank resistance of hydraulic radius segmentation method，segment the bed rock surface roughness，and obtain the relationship of class evenly spread gravel dense roughness ns and average constant volume size .The results show that the representative element weight of four-grades homogeneous gravel is 1 kg to 2 kg，and the value can be determined according to the particle size；ns=0.013 91/6V.