Based upon the fundamental principles of fluid mechanics, a dimensionless differential equation of motion for the side channel flow in an elevated water tank for the water flow-calibrating system and its numerical solutions under different conditions is presented in this article.The results of computation of the surface profiles of the side channel flow are basically in agreement with the experimental data. By virture of them, the authors have developed the procedures of engineering designing and practical formulas for both level and sloping bottom side channels. As a continuation arid supplement of literature[1] and[2], the paper has completely solved the designing problem of the surface structure of the elevated reservoirs in water flowrate calibration facilities.