Water conservancy projects are critical national infrastructure, and their quality and safety are directly related to the national economy and people‘s livelihood. To investigate the distribution characteristics and causes of quality behavior problems and entity quality problems in the quality supervision of large-scale water conservancy projects, this study takes a large-scale water conservancy project on the Han River as the research object. Based on the inspection and supervision data collected by the on-site quality supervision project station from 2019 to 2025, a systematic classification, statistical analysis, and cause analysis are conducted. The results show that quality behavior problems are mainly concentrated in three aspects: quality management system, quality process control, and quality evaluation and acceptance, while entity quality problems are most prominent in concrete works. The causes include differences in inspection coverage and problem identification, a management tendency to prioritize entity quality over behavior quality, and relatively outdated supervision methods. Accordingly, countermeasures are proposed, including clarifying the rights and responsibilities of the project legal person, strictly controlling survey and design quality, standardizing the performance of supervision contractors, consolidating construction management responsibilities, improving the quality inspection system, and strengthening the capacity building of quality supervision institutions. This study can provide empirical reference for quality supervision and quality management of similar large-scale water conservancy projects.