Chemical corrosion in coal mines substantially affects the mechanical properties of rocks such as sandstone and may trigger the instability of roadways. In this article, the effects of chemical corrosion on the Mode I fracture toughness of sandstone are investigated using semicircular bending tests. The two experimental factors involved in this article are the pH value of the soaking solution and the soaking time. Based on the peak load and the shape parameters of specimens, the Mode I fracture toughness of sandstone under chemical corrosion is estimated as a function of the pH value of the soaking solution and the soaking time. In soaking tests, the relationships between the pH value of the soaking solution and the soaking time and the relative quality and porosity of sandstone are determined. The results indicate that chemical corrosion weakens the mechanical properties of sandstone specimens but enhances the ductility of sandstone specimens. The Mode I fracture toughness of sandstones decreases with an increase in the soaking time. The Mode I fracture toughness of sandstones initially increases but then decreases with an increase in the pH value. When the pH value is 7, the Mode I fracture toughness of sandstone attains the maximum value. The damage to sandstone specimens shares the same mechanical mechanism as the deterioration of the Mode I fracture toughness of sandstone. The results are significant for establishing a coupling damage model for the mechanical–hydrochemical behaviors of rocks.