The influence of slag and aggregate volume content in concrete on the degradation, linear expansion, mass change and relative elastic dynamic modulus of concrete exposed to 424g/L MgSO4 solution were investigated. The microstructures of sound and deteriorated concrete were also determined using backscattered electron imaging analysis(BSE IA), energy dispersive X ray spectroscopy analysis(EDS) and mercury intrusion porosimetry(MIP). Results show that the addition of slag is able to refine the pore structure and increase the impermeability of corresponding cement based materials. However, the incorporation of high amount of slag exerts an adverse effect on magnesium sulfate resistance of concrete by reducing calcium hydroxide, promoting the decalcification of C S H and the formation of M S H. It is found in BSE EDS analysis that the preferable deposition of gypsum in the region close to aggregate could be the reason for the severe degradation of specimens made with a relative high amount of aggregate at a high concentration of magnesium sulfate. Therefore, the mix design of concrete which is supposed to serve in the magnesium sulfate environment should consider the influence of porosity, compositions of cementitious materials and those of hydration products in a comprehensive way.