The Environmental Protection Tax Law （EPTL） is an important driving force to promote green innovation of enterprises. Based on the environmental legitimacy theory and resource dependence theory， adopting unbalanced panel data of listed companies in Chinese A-share market from 2007 to 2022， and taking the implementation of the Environmental Protection Tax Law as a quasi-natural experiment， it used propensity score matching and differences-in-differences （PSM-DID） method to investigate the effect of EPTL on green innovation. Additionally， it examined the chain mediating effects of environmental legitimacy， green subsidies and financing constraints， as well as the moderating effect of market monopoly. The results show that the pressure and incentive mechanisms of EPTL play a promotive role in enterprise green innovation； the EPTL facilitates enterprises in obtaining environmental legitimacy. Enterprises possessing such legitimacy can acquire green subsidies and ease financing constraints， thereby indirectly promoting green innovation. Market monopolies hinder the positive impact of the EPTL on enterprise green innovation. The conclusion not only provides a new perspective on how the EPTL affects enterprise green innovation， but also offers important insights for legislative bodies to refine EPTL， for government departments to curb market monopolies， and for enterprises to obtain environmental legitimacy.