Carbon emissions trading (CET) is now one of China’s key policy tools for achieving the goals of carbon peak and carbon neutrality. To comprehensively explore the consequences of China’s CET policy, the study first evaluated the content validity of CET policies across China’s 31 provinces, autonomous regions, and municipalities falling directly under the central government using policy strengths, tools, and measures from 2011 to 2020. The spatiotemporal drivers of regional carbon efficiency improvements from aspects of actual policy effect, average policy content validity effect, and policy quantity effect were also evaluated via the logarithmic mean Divisia index. This analysis revealed that the policy content validity was increasing in general and was higher in pilot regions. On average, the policy quantity effect was the primary driver of carbon efficiency improvements in both pilot and non-pilot regions, but the actual policy effect failed to promote carbon efficiency in both regions. Beijing’s carbon emissions reduction effect was superior to that of other pilot regions, and where actual policy effect and policy quantity effect were the primary and secondary drivers of local carbon efficiency improvements, respectively. These findings suggest that when formulating CET policies, each region should not only focus on improving policy content validity, but also pay attention to the actual carbon emissions reduction effects produced by policies as well.