Effectively addressing overcapacity is an essential pathway for achieving quality transformation, efficiency improvement, and growth momentum conversion in China's modernization process during the 15th Five-Year Plan period. This study constructs a mechanism indicator of enterprise strategic ambidexterity based on the closed-loop framework of "R&D-commercialization" and employs micro-level data from Chinese A-share listed firms during 2008-2024 to empirically examine the impact of national unified market construction on enterprise capacity utilization rate. The results show that national unified market construction significantly improves enterprise capacity utilization rate, and the findings remain robust after a series of robustness tests. Mechanism analysis indicates that national unified market construction enhances enterprise capacity utilization rate by expanding market demand and promoting enterprise strategic ambidexterity. Heterogeneity analysis shows that the positive effect of national unified market construction on enterprise capacity utilization rate is more pronounced among firms located in regions with better institutional environments and among enterprises with abundant patient capital and higher levels of digital-intelligent transformation.Further analysis demonstrates that global value chain position and involutionary competition exert negative moderating effects on the policy effectiveness of national unified market construction. These findings provide policy implications for mitigating involutionary competition, deepening market-oriented factor allocation reforms, and promoting the integrated development of domestic and international dual circulation.