In order to clarify the erosion damage mechanism of CA mortar in simulating acid rain, specimens were immersed in acid solution with pH of 30, 40 and 50 Compressive strength tests, differential thermal analyzer, scanning electron microscope with energy dispersive spectrometer as well as biological microscope were applied to analyze the erosion damage mechanism of CA mortar in simulating acid rain. The results show that long term immersion in acid solution causes the specimens to crack, spall and decrease of compressive strength. The quantity of C S H gels in the specimen reduces from the inside out, while lots of gypsum crystals are found on the surfaces of the specimens under above mentioned conditions. Whats more, lots of filamentous fungi and yeasts are contained in long term immersing acid solution, which is consistent to the situation of seriously biological damage of CA mortar on the scene. The destruction processes of CA mortar in acid solution include C S H gel dissolution process, gypsum crystallization expanding process and biological corrosion process.