The lug is the basic element of a paddy-field lugged wheel. Soil flowed under the lug develops the pull and lift forces on the lug. In a set of specially designed soil box, the soil flow under the lug was photographed and the soil reaction forces on the lug were measured. Firstly, soil flow track, flow mode and flow zone under a lug were studied. The effects of the pull force, lift force and driving efficiency and their relationship with the soil flow in the interaction between the soil and lug were analyzed. The cause of low driving efficiency was further studied for the lug with slip and deep sinkage. 