The safety of the existing structures,flow conditions,construction conditions and investment should be considered if building a new ship lock next to the one already built.Taking Honghua second-line ship lock for example,we analyze the layout of the existing hydropower complex and characteristics of the existing structures,and determine the construction object of the second-line ship lock.The principles and methods of site selection and planning are expounded,and technical and economic comparison is conducted among different schemes,which may serve as reference for similar projects.