The diurnal and spatial variations of urban heat island (UHI) and their influence factors in Beijing central business district (CBD) are investigated by using the dense air temperature data measured in this area during 2012-2013. The results illustrate that the CBD air temperature was 0.64℃ higher than the surrounding area. And the UHI in the CBD was stronger in the nighttime and weaker during the daytime, while the "urban cold island" even occurred at the noon. Analysis of seasonal means of the UHI shows that the strongest UHI in the nighttime occurred in the autumn, followed by that in the winter, and the weakest UHI occurred in the spring and summer. During the daytime, the strongest UHI occurred in the summer, followed by that in the winter and the weakest UHI was found in the spring and autumn. Foggy, rainy, and windy days could restrain the UHI compared to the situation in sunny days. Combined with the wavelet analysis results, the reason why the UHI in the autumn was stronger than in the winter is attributed to the high occurrence frequencies of foggy, rainy, and windy days in the winter. Analysis of the UHI spatial variation in the CBD shows that gardens, schools and other public lands could ease the UHI effect. And the standard deviation of spatial variation of the UHI in the CBD in foggy, rainy and windy days was smaller than that in sunny days.