Vegetation plays a critical role in climate regulation by cooling and humidifying urban regions. The climate regulation capacity of vegetation varies in different seasons, and its association with the coupling of temperature and humidity remains poorly understood. Here, we investigated the climate regulation potential of vegetation by establishing a set of indices, namely, cooling intensity (CI), cooling efficiency (CE), humidifying intensity (HI), and humidifying efficiency (HE), using continuous decadal field measurements (2013–2022) of temperature and humidity in vegetation and built-up areas in Beijing, China. Additionally, we examined the coupling effect and threshold of cooling and humidification using these indices. Climate regulation by vegetation was insignificant at night. However, the daytime results were as follows: (1) the maximum CI and CE were similar in four seasons, and their ratio was 2:3:2:1 in spring, summer, autumn, and winter, respectively; (2) the ratio of maximum HI was 4:10:7:3, whereas that of the maximum HE was 7:8:7:2 in spring, summer, autumn, and winter, respectively; and (3) the coupling of ambient temperature and humidity affected the climate regulation of vegetation. The CE and HE of vegetation increased significantly when the ambient temperature exceeded 15.1 °C and 8.5 °C, respectively. The CI and HI decreased with increasing humidity and were highest in a high temperature and low humidity ambient environment. Our study presents scientific insights into vegetation cooling and humidifying effects under different climate backgrounds and provides an enhanced understanding of the coping ability of vegetation in a severe heat island environment.