<p>The tranquil environment of high-density urban residential areas had an important impact on the quality of life of residents, but low sound pressure level didn’t necessarily lead to a tranquil experience. The purpose of this study was to explore the constituted and affected factors of tranquility in high-density urban residential areas. Nine high-density urban residential areas were selected, and 216 questionnaires of evaluations on tranquility, the sound sources and environmental indicators of residential areas were collected. Principal component analysis, correlation analysis and multiple linear regression were used to analyze the data. The results showed that perception of restorativeness (72.61%) and pleasantness (27.39%) constituted the evaluation of residential tranquility. Natural sound sources of wind, leaves, and birds promoted restorativeness and pleasantness of tranquility, while selling loudspeakers, mechanics, and traffic reduced restorativeness and pleasantness of residents. Meanwhile, many environmental factors, such as public infrastructure, landscape greening, and security affected the tranquility, except public spaces or cleanliness. Moreover, richness of plant configuration was not a factor which affected restorativeness of tranquility evaluation either in this study. This study constructed the evaluation system of residential tranquility, clarified the mechanism of sound source and environmental factors, and provided theoretical basis and practical guidance for soundscape optimization and environmental design of high-density urban residential areas. It is suggested to integrate natural soundscape, add noise reduction facilities, optimize greening configuration and improve environmental quality in residential area planning, so as to enhance residents’ restorativeness and pleasantness, and promote sustainable development of community.</p>