China is considering joining the CPTPP to promote deeper reforms and higher-level opening-up policies. This paper constructs a structural model incorporating tariff and non-tariff barriers to analyze the theoretical impact of trade barrier reduction on the national economy. Through counterfactual analysis, we assess the welfare effects of CPTPP, its structural decomposition, and evaluate China’s potential response plans. The results show that CPTPP mainly impacts welfare through trade volume effects, improving member countries’ welfare while causing welfare losses for non-members due to trade diversion. China’s welfare impact is relatively small. Further analysis indicates that joining the CPTPP would benefit China and strengthen trade ties, enhancing welfare for both China and member countries. Additionally, China can offset potential negative impacts by advancing initiatives like the Belt and Road, RCEP, China-ASEAN Free Trade Area, and BRICS Alliance to a higher level of openness.