Salt has a significant effect on the moisture absorption isotherm and diffusivity of porous building materials, thus affects the heat and moisture transfer. However, the influence of sodium sulfate (Na2SO4) on these properties of sintered brick has rarely been studied. In this study, the isothermal moisture absorption and moisture diffusivity (Dl) of Chinese sintered blue bricks with different Na2SO4 content was tested. The Dl was determined by a simple method (called ‘ruler method’) based on capillary absorption experiment. Then using the measured Dl, the capillary absorption process of sintered bricks with different salt content was reproduced based on the coupled heat and moisture transfer model. The results show that the equilibrium moisture content (EMC) of sintered blue bricks dramatically increased at the relative humidity (φ) higher than 80% due to the presence of Na2SO4, especially in extremely high relative humidity (more than 94%). The EMC values of bricks with 1.74% Na2SO4 content was 12.0 times higher than that of salt free bricks under 99% relative humidity. As the Na2SO4 content increased, the capillary absorption coefficient of sintered bricks decreased with maximum decline of 28.7% while the capillary moisture content was basically unchanged. The Dl obtained from the ruler method increased in low moisture content range and decreased in high moisture range with the increase of salt content, and can simulate the capillary absorption rate reasonably well. This study provides basic data for coupled heat, moisture and salt simulation, energy calculation and heritage conservation of extensive architectural heritage envelopes.