Enhancing the spatial vitality of coastal cities is beneficial for the sustainable development of urban construction. However, how to fully utilize coastal resources and boost urban vitality is an important issue. This study takes the coastal city of Bayuquan in China’s cold region as an example. Firstly, we conducted field investigations and data mining in Bayuquan, utilizing Baidu heat map data to measure the spatial–temporal vitality of different areas in Bayuquan. Secondly, we used Moran’s I test to examine the spatial autocorrelation of coastal spatial vitality. Lastly, with the help of the OLS and GWR models, we explored the factors influencing spatial vitality and the urban built environment. The research findings indicate the following: (1) There are spatial–temporal differences in the vitality of different areas in Bayuquan, heavily influenced by the tourist season. (2) The OLS results show that the impact of the built environment on spatial vitality exhibits spatial heterogeneity during different tourist seasons. However, we found no spatial heterogeneity in the influencing factors in the harbor district. (3) The harbor district and the tourism-driven district re quire differentiated construction guidance. Facility functions and block morphology mainly influence the vitality of the harbor district, while the vitality of the tourism-driven district is primarily affected by its aesthetic characteristics. This study can propose differentiated regional construction guidance and specific feasible coastal urban design strategies for seasonally influenced coastal city construction. It holds significant implications for improving urban living quality and is vital to urban decision-makers, planners, and stakeholders.