Experiment of water+CO₂+sandstone system in the autoclave reaction was conducted at different temperatures (100 ℃,150 ℃, 200 ℃). Polarized light microscopy, technical means of the sweep surface electron microscopy, X-ray diffraction analysis and UV spectrophotometer were used to study the effect of CO₂ fluid transformation activities on the reservoir properties. Secondary porosity develops in the reservoir sandstones duo to the dissolution resulted from dissolved CO₂ fluid. Dissolution occurs mainly in the position of low energy and lattice defects. This study is of great significance on the Numerical simulation and evaluation of geological storage of carbon dioxide clastic reservoirs.