Through the experimental study on the model of the single-box three-rooms double-deck concrete simple box girder, analyzed the shear lag of the box girder under the double uniformly distributed loads of three different conditions. The results showed that: the shear lag effect on the roof of midspan section and 1/4 cross-section both showed positive shear lag phenomenon; the shear lag effect has nothing to do with the size of the load but with the location of the load under elastic stage；when the loads applied on the base plate，the shear lag effect are smaller than the situation that the loads applied on the upper plate ；when transferring part of the load from top plate to the bottom plate，the shear lag effect of box girder will reduce.