In order to identify the slaking characteristics of mudstone in different pH solutions and provide some theoretical basis and technical guidance for utilization of mudstone in areas polluted by acid rain,some laboratory tests of slaking were performed on mudstone from the open-pit mine of Haizhou in Fuxin. The slaking phenomena in different pH solutions and changes in grain contents and slaking ratio of mudstone samples in slaking process were analyzed. Through X-ray diffractometer(XRD)and fluorescence spectrum analysis system，the mineral & chemical composition of mudstone were made clear. The test results showed that the rank of these samples as slaking degree was acid>alkali>neutral. With an increase in the number of cycles，the slaking degree decreased while the slaking rate increased at first two cycle，and then it gradually slowed down. The slaking of mudstone in the acid solutions was the most severe. The fundamental reason was that most of the mineral composition of mudstone could not dissolve in water but chemical reaction between it and acid solutions produced soluble salts. It was significant to take into account the acidity of mudstone's surroundings in engineering construction.