Abstract: To explore its degradation laws under the action of drying-wetting cycles in a chemical solution, the Ⅰ/Ⅱ mixed-mode fracture toughness of sandstone is obtained through the semi-circular bending tests (SCB) after experiencing 1, 3, 6 and 10 times of drying-wetting cycles in immersion solution of pH equal to 7, 9 and 4. The test results show that with the increase of the times of drying-wetting cycles, the deterioration degree of fracture toughness of sandstone gradually increases, and the deterioration rate gradually slows down. In an acid environment (pH=4), the deterioration degree is the largest, followed by that in alkaline environment and that in neutral environment. The fracture toughness of mode Ⅱ is less degraded than that of mode Ⅰ. Then, based on the test results, a large error is found when using the maximum tangential stress (MTS) criterion to identify the composite crack dominated by mode Ⅱ fracture, while the generalized maximum tangential stress (GMTS) criterion can better match the test results. Finally, based on the GMTS criterion, the degradation characteristics are analyzed. With the increasing degradation degree of fracture toughness, the absolute value of T-stress and the critical polar radius r_c gradually decrease. The moisture condition of samples has almost no effects on r_c, which confirms that r_c is a parameter reflecting the structural quality.