To investigate the geochemical dissolution processes and controlling mechanisms of clastic minerals in sandstone reservoirs of the South Yellow Sea Basin following CO2 injection, this study conducted CO2-water-rock reaction simulations at different temperatures (53°C, 73°C, 93 °C) under original formation pressure conditions using sandstones from the Longtan Formation of the Upper Permian at the CSDP-2 well. Utilizing a self-developed high-temperature, high-pressure reactor and a high-fidelity sampling system, the study performed geochemical testing and electron probe microanalysis on obtained fluid samples and rock thin sections. This systematically revealed the influence of temperature and mineral composition on the CO2-water-rock reaction process, pore structure evolution, and ion migration behavior. Results indicate that temperature is the key factor regulating mineral dissolution and secondary mineralization pathways. At 53 °C, carbonate minerals dissolve significantly; at 73 °C, mineral dissolution coexists with new mineral precipitation, with pores predominantly distributed as dispersed micropores; at 93 °C, mineral dissolution is most intense, accompanied by significant secondary mineral precipitation, leading to fractured and locally blocked pore structures. Different ions exhibited distinct temperature response characteristics: Na+ and K+were controlled by silicate weathering and passivation films; Mg2+ continued to release at high temperatures; Ca2+demonstrated a temperature-dependent dissolution-precipitation dynamic equilibrium. The study indicates that marine sandstones undergo multiphase reactions during CO2 injection, primarily involving carbonate dissolution, clay alteration, and slow silicate decomposition, with the pore structure exhibiting an “expansion-reconstruction” evolutionary pattern. Temperature and mineral composition jointly regulate the dissolution-precipitation equilibrium, serving as key factors influencing reservoir porosity-permeability characteristics and CO2 sequestration behavior.