In this study, we aimed to reveal the impact of the urban heat island effect on plant phenology in a mountain city, To this end, ground observations of the phenology of 80 woody plants were conducted from January 2016 to January 2017 in the main urban area of Chongqing, and Landsat 8 thermal infrared data as well as concurrent temperature data were used for classification of heat island intensity. The result showed that, compared with a cool island, the urban heat island has led to an advance in the transition zone and heat island zone for spring phenology (5.1 and 8.1 days for the timing of leaf expansion, 4.0 and 20.8 days for the timing of blossoming, and 4.8 and 11.6 days for the timing of flower fall, respectively), and a delay for autumn phenology (8.5 and 18.9 days for the timing of leaf fall, respectively). Furthermore, the growth season has been extended. Differences existed among functional types in response to urban heat island effect; the sensitivity of shrubs, evergreens and exotic plants to urban warming were found to be higher than that of trees, and deciduous and native species. Our research, to some extent, fills the gap in the research on phenology in the southwest area of China and might provide an early warning for predicting the phenological responses of urban plants to future urban microclimate changes in mountain cities and even to global warming trends.