<p>The mechanical properties of the interaction between brine, CO₂ and sandstone are very complex, so the quantitative evaluation of their tensile properties and acid–base properties is very important for engineering. In order to study the changes of tensile properties and acid–base properties of red sandstone under different brine and CO₂ immersion times, BST, XRD and nano-indentation techniques were used to analyze the changes of tensile properties and microstructure of red sandstone under different soaking times and PH. The results show that the PH value of the solution decreases from 8.25 to 6.64 due to the dissolution of carbon dioxide, and the acidic solution dissolves the brittle minerals such as calcite, while secondary calcite is produced, and the micro-cracks inside the rock become more tortuous, resulting in the deterioration of mechanical properties. The lower the PH value, the higher the dissolution rate and the greater the degree of crack filling. High temperature makes secondary calcite regenerate calcite precipitate, release carbon dioxide, PH value rises, the solution becomes weakly alkaline, part of the pores of the rock are filled, cracks will become straight, resulting in an ‘I’ crack pattern, leading to mechanical properties strengthening. When carbon dioxide dissolves calcite twice, the PH value decreases slightly, the porosity of the rock increases slightly, and the zigzag degree of the micro-crack increases slightly, and a partially filled ‘)’ type crack mode is obtained, which deteriorates its mechanical properties. The results confirm the mechanism of mechanical, chemical and temperature influence of sandstone in brine and CO₂.</p>