There is a significant difference between the degree of degradation of blue bricks in historic building of Taiyuan and that of Fuzhou. In order to study the effect of sorption properties of blue bricks on this difference, an experiment based on the national standard (GB/T 20312—2006/ISO 12571:2000) to test the isothermal sorption properties of the blue bricks in historic building of Taiyuan and Fuzhou with a view to comparing the difference between them was designed. The results indicate that, with the relative humidity ranging from 7%92%, the equilibrium moisture contents of the blue bricks in Taiyuan are between 0002900291kg/kg, while those in Fuzhou are only between 0001300024kg/kg. When relative humidity varies in a higher region, the hygroscopic moisture content of blue bricks in Taiyuan is 612 times more than that of the blue bricks in Fuzhou. It suggests that the blue bricks in Taiyuan have greater hygroscopicity than those in Fuzhou, and therefore are easier to be influenced by the humidity of environment. The higher water sorption content of blue bricks in Taiyuan is liable to deteriorating, which is one of the causes that lead to higher degree of degradation of blue bricks in Taiyuan than that in Fuzhou.