The rapids abating hydraulic index generally refers to the combination of the maximum route flow velocity and gradient of a ship which can sail through the rapids by itself under the condition of rated load and rated main engine power. It is the critical flow condition to determine whether the ship can sail through the rapids by itself, and it is the reference basis for analyzing the effect of rapids regulation. This paper analyzes the defects of the existing methods to determine the rapids abating hydraulic index, and points out the shortcomings of the existing expressions of comprehensive rapids abating hydraulic index, such as unreasonable selection of key parameters, unclear relationship between influencing factors and main variables, etc. Based on the basic relationship between flow resistance and flow velocity, gradient resistance and gradient, and according to the basic principle of the balance between ship thrust and navigation resistance, from the perspective of overcoming the existing shortcomings of the calculation and expression of the rapids abating hydraulic index, a traditional expression and pre-estimation formula of dimensionless rapids abating hydraulic index were derived. Applying the test results of 500-ton ship at hydrostatic speed in Lancang River, the variation law between the ratio of actual propeller speed to the rated propeller speed with the advancing coefficient was obtained. When calculating the rapids abating hydraulic index, the propeller speed should not be taken as the rated speed. The results show that there is an obvious deviation between the flow resistance calculated by the Zvankov formula and the ship thrust. The correction curve of the resistance correction coefficient with the ship Froude number was plotted. It is pointed out that the calculation formula of Zvankov flow resistance needs to be properly modified according to the results of full-scale ship test. The rapids abating hydraulic indexes of various types of motor ships in Lancang River were analyzed and calculated, and the rationality of the traditional expression of dimensionless rapids abating hydraulic index were verified. It is proved that the dimensionless parabola is more appropriate to describe the relationship between flow velocity and gradient in the rapids abating hydraulic index. Based on the principle of dimensional analysis and through the numerical regression analysis of the rapids abating hydraulic indexes of various motor ships in Lancang River, the pre-estimation formula of dimensionless rapids abating hydraulic index was established, which can significantly improve the calculation efficiency of the rapids abating hydraulic index.