Abstract: Specific energy is a foremost hydraulic parameter of stepped spillways, a measure of kinematic energy of moving water. This paper compares specific energies between stepped spillway and smooth spillway, and works out a new concept—relative specific energy that describes the difference of head loss between the two spillways. Model tests on skimming flows of 21 sets were conducted using a spillway slope range of 26.6°~48° and a step height range of 0.5~2.0 m, focusing on the variations of specific energy and relative specific energy with the length of flow path, unit discharge, step height, and bottom slope. The results indicate that the specific energy varies with these parameters in a manner too complicated to apply practically while the relative specific energy that is independent of discharge, varies linearly with the length of flow path. Step height has a great influence on the relative specific energy that increases with it and spillway slope. The results prove the necessity of stepped spillway analysis adopting relative specific energy and its advantage in practical applications. An empirical formula for calculation of water head loss is given.