The urban heat island (UHI) effect has had an important impact on building energy demand in big cities. It is of great significance, therefore, to evaluate the impact of the UHI on building energy demand in a metropolis in order to save energy for existing buildings and to better design for future buildings. Based on the hourly data from automatic weather stations and four representative rural weather stations selected by satellite remote-sensing method, the impacts of urban heat island intensity (I_UHI) on building heating and cooling loads of different time scales (annual, daily, and hourly) in Tianjing City were evaluated by simulating hour-by-hour loads of the typical residential buildings. The results show that: 1) The heating loads decreased and the cooling loads increased with the increase of the I_UHI in the past nine years. The decreased magnitude of the yearly heating load was larger than the increased magnitude of the yearly cooling load. When the I_UHI increased by 1 ℃, the annual mean heating loads decreased by 4.01 kWh/m² and the annual mean cooling loads increased by 1.05 kWh/m² in the urban areas compared to the rural areas. 2) The heating loads were the highest in the period from late December to late January of the following year, and the daily mean loads in the urban areas were about 10% lower than that in the rural areas. The cooling loads were the highest from late July to early August, and the daily mean loads in urban areas were about 6% higher than that in the rural areas. 3) Both heating and cooling loads were higher during the nighttime than daytime. The period from 18:00 to 07:00 Beijing Time was the high load stage in the urban and rural areas, regardless of heating period or cooling period. From 11:00 to 15:00 Beijing Time, however, the loads were low in the heating period but high in the cooling period, which may have been caused by changing temperature and the different heating or cooling demand of the urban residents. Therefore, the impact of UHI on hourly and daily building loads should be fully considered to promote the effective management of urban heating and cooling operation for reducing building energy consumption in big cities of northern China like Tianjin.