A large number of existing bridges have insufficient navigation clearance in China’s high-grade inland waterways. How to scientifically and rationally determine the standard of step-by-step reconstruction is a universal and key technical problem to be solved urgently in the construction of the high-grade waterway. Taking the Beijiang and Dongjiang waterway in Guangdong province as examples，we analyze the clearance dimension of the existing bridges in the study area. Combining with the prediction of representative ship types，we put forward the requirement of bridge clearance dimension for ship navigation and analyze the adaptability of the flood standard of the designed maximum navigable water level. It is recommended that the clearance dimension of the bridge should meet the requirements of 8.3 m clear height （ the flood recurrence period of the design maximum navigable water level is 2 years），and 86 m clear width of two-way navigation with single holes，or 44 m clear width of one-way navigation with double holes in the Beijiang and Dongjiang kiloton waterway projects. On this basis，taking into account the experience of other waterway constructions，suggestions are put forward to adapt to the reconstruction standard of the kiloton mountain waterway bridge. The conclusion may serve as a reference for the construction of the high-grade inland waterway.