Urban tourism imagery is an essential factor affecting the competitiveness of cities. However, most existing studies use small data methods such as interviews and questionnaires to explore tourists’ perceptions of cities without combining big data for analysis. In addition, there is relatively little research on the tourism imagery of coastal cities. Therefore, this study analyzes the data from 523 travelogues from a three-dimensional perspective of identity, structure, and meaning, using methods such as BERTopic, aiming to explore the tourism imagery of coastal cities perceived by tourists. The results show the following: (1) In terms of identity, Sanya’s urban tourism imagery has the attribute of “external explicit-internal implicit”, which satisfies tourists’ visual and spiritual experiences. (2) Regarding structure, Sanya has a clear regional division of imagery and a core–edge diffusion distribution among regions. (3) Regarding meaning, tourists’ attitudes toward Sanya are polarized. Positive emotions predominate in the overall perception of meaning, but 25% of the negative emotions require urgent attention. This study provides a reference for the sustainable development of tourism cities and the marketing management of destinations.