Urban green spaces (UGSs) can help cities achieve high-quality and sustainable development by regulating local climate through the thermal effects. A large number of studies have investigated the spatial pattern of the summer cooling effects of UGSs, while few studies explored the temporal pattern and underlying factors of UGS thermal effects on an annual basis. This study selected seven representative urban parks in Nanjing City to investigate their yearlong thermal-effect dynamics and underlying meteorological factors. Air temperature sensors were mounted on pole-like structures at around 2.5m above the ground at both the center and boundary of the parks. To assess the thermal effects, fixed measurements were also taken at two urban reference sites in the central business district that has a compact high-rise morphology. The results indicated significant differences in the seasonal and daily dynamics of thermal effects between the seven parks due to their differences in vegetation coverage, canopy closure and altitude. Three temporal patterns of thermal effects were discovered, including cooling throughout the year, daytime warming and nighttime cooling, and daylong cooling in summer and daytime warming nighttime cooling in other seasons. Compared with the urban center, the park could lower the air temperature by up to 7.7℃ in summer and increase the temperature by 3℃ in winter. However, the impacts of urban parks on urban climate were not always beneficial, since they can display a warming effect in summer daytime and a cooling effect in winter nighttime. The four background meteorological factors, including air temperature, relative humidity, wind speed and solar radiation, could explain 23.5%-77.4% of the variations in the thermal effects. Air temperature and solar radiation were the primary factors for daytime thermal effect, while wind speed mainly affected the nighttime thermal effect. For every 100W/m² increase in solar radiation, the summer daytime cooling effect could decrease by 0.3℃, and the winter daytime warming effect could increase by 0.2-0.5℃. For every 1℃ increase in air temperature, the summer daytime cooling effect could increase by 0.1-0.14℃, and the winter daytime warming effect could increase by 0.05-0.08℃. For every 1m/s increase in wind speed, the summer nighttime cooling effect could reduce by 0.05-0.78℃, and the winter nighttime cooling effect could decrease by 0.4-1.1℃. The above results revealed the complex formation mechanism of the thermal effects of urban green spaces, and might provide scientific references for natural conservation and climate adaptative design of urban parks in hot summer and cold winter areas.