Abstract: In the new stage of ecological civilisation construction, giving full play to the important role of each subject is the realistic requirement of planning, deploying and promoting the synergistic effect of pollution reduction and carbon reduction as a whole. The article starts from the perspective of environmental subjects, based on the real dilemma of insufficient governance motivation and unsystematic governance supervision of environmental subjects, and on the basis of systematic interpretation of environmental legislation affecting the synergistic effect of pollution reduction and carbon emission reduction in the context of administrative supervision, the article takes the Air Pollution Prevention and Control Regulations and the policy of environmental protection supervision as the impact of the event, and empirically examines the impact of environmental legislation on synergistic effect of pollution reduction and carbon emission reduction and its inherent mechanism in the context of administrative supervision by adopting the panel data of 282 cities in China in the years 2004-2021. The impact of environmental legislation on the synergistic effect of pollution reduction and carbon reduction and its internal mechanism. It is found that: (1) environmental legislation has a significant synergistic effect on pollution and carbon reduction, i.e., it suppresses environmental pollution and carbon dioxide emissions while enhancing the synergistic management efficiency of pollution and carbon reduction; (2) the analysis of the moderating effect shows that administrative supervision and environmental legislation have a more significant positive moderating effect, i.e., the effect of environmental legislation on suppression of environmental pollution emissions and enhancement of the synergistic management efficiency in the context of administrative supervision is more significant but the effect on CO carbon dioxide is less obvious; (3) Heterogeneity effect analysis shows that environmental legislation has significant temporal heterogeneity characteristics. Specifically, from 2004 to 2012, environmental legislation mainly shows the enhancement effect on the synergistic governance efficiency of pollution reduction and carbon reduction, and from 2013 to 2021, it can not only significantly enhance the synergistic governance efficiency of pollution reduction and carbon reduction, but also significantly inhibit environmental pollution and carbon dioxide emissions; (4) Mechanism analysis shows that green innovation and emerging industry development are important paths for environmental legislation to achieve the synergistic effect of pollution reduction and carbon reduction. Therefore, the article puts forward policy suggestions from accelerating the process of local environmental legislation, expanding the scope of administrative supervision, and increasing the support for green innovation of enterprises.