Based on the daily observations of Beijing's 20 conventional meteorological stations from 1976 to 2015 and the urban development data of Beijing in the same period, the distribution and variation characteristics of relative humidity in Beijing and its response to urbanization are analyzed by using a new classification method of representative stations in urban and suburban areas. The results show that the spatial distribution of relative humidity in Beijing is uneven, in which the spatial distribution of relative humidity is mainly affected by topography in the early stage of urbanization and by urbanization in the stage of higher urbanization. In recent 40 years, the average relative humidity in Beijing presents a downward trend with a linear tendency rate of -0.9%·10 a~(-1), and the decline rate of urban relative humidity is larger than that of suburbs especially after 1990s. Moreover, the intensity of urban dry islands in Beijing shows an upward trend in the past 40 years, with a linear tendency rate of 1.3%·10 a~(-1). The urban dry island effect is the strongest in winter, followed by autumn and spring, and weaker in summer. Note that the dry island effect of Beijing has shown a more obvious trend of enhancement since 1995. Additionally, the urban dry island effect of Beijing is closely related to the urbanization process, and has a significant high correlation with the urbanization rate and the intensity of urban heat island, with the correlation coefficients as high as 0.87 and 0.86 respectively.