Old oilfields in China play a crucial role in ensuring national energy security. However, challenges such as high water content, strong reservoir heterogeneity, and the dispersed nature of remaining recoverable reserves limit oil recovery efficiency. Controlled phase transition polymers (CPTPs), as an improved chemical flooding agent, are designed with weak crosslinking to minimize viscosity shear loss during injection. These polymers undergo phase transition in reservoir conditions to form high-viscosity solutions, enabling profile control and expanded sweep volume, thereby significantly enhancing oil recovery. In this study, quantum chemical calculations and molecular dynamics simulations were employed to investigate the reaction kinetics and viscosity variation mechanisms of CPTPs during phase transition. The results reveal that the bond dissociation energy of the ester C–O bond is lower than that of other reaction sites, indicating higher phase transition potential. Among four newly developed crosslinked polymers, the phthalic acid diacrylate crosslinked polymer demonstrated the lowest reaction energy barrier and significant viscosity enhancement, making it the most promising candidate for practical engineering applications.