In this study, we explore the inverted U-shaped association between internal control quality and firm operational efficiency. Although effective internal controls can facilitate and improve operational efficiency, excessive internal controls can negatively affect operational efficiency by (1) influencing management energy, attention, risk-taking, and innovation motivations, (2) hindering employees' creativity, enthusiasm, and trust. Our findings support the inverted U-shaped association. We further explore and prove the two channels through which internal controls affect firm operational efficiency: the “information channel” (the quality of internal management reports), and the “application channel” (the enforcement of internal controls). Additionally, we show that the inverted U-shaped association only exists in non-state-owned firms. We do not find significant association between internal control quality and operational efficiency in state-owned firms. Overall, this study suggests that firms should not only establish an optimal level of internal controls, but also enforce the internal controls effectively to achieve their intended goals.