River basins are the basic geographical units that maintain the harmonious coexistence between humanity and nature, as well as major spatial carriers for economic and social development.At the pivotal juncture when Chinese modernization is comprehensively advancing harmonious coexistence between humanity and nature, empowering the green and high-quality development of river basins by deepening the reform of fiscal and taxation systems is a major issue that urgently requires further study during the “15th Five-Year Plan” period.Guided by the Two Mountain Theory, drawing on foundational theories such as Pigouvian tax and Coase’s theorem, this paper integrates the characteristics of the “nature-economy-society” complex ecosystem in river basins to construct a collaborative analytical framework that coordinates “development-security”, integrates “institutions-markets”, and strive for the coupling of “humanity and nature”. The study finds that, the core problem in current fiscal and tax systemin river basin governance is the coupling failure between government regulation and market mechanisms, which hinders the value realization mechanism of ecological products, a hallmark of modernization.Specifically, this failure is reflected in four aspects: the tax system design fails to adequately respond to the holistic nature of the complex ecosystem; policy tools fail to effectively activate the synergy of diverse stakeholders; governance structures fail to fully adapt to the spatial fluidity of river basins; and supervision and evaluation fail to precisely align with the dynamic nature of fund operating cycles in river basins.Based on these fndings, guided by the goal of comprehensive green transition proposed at the Fourth Plenary Session of the 20th Central Committee of the Communist Party of China, this paper grounds itself in the institutional endowments required by Chinese modernization, draws fully on international experience and proposes reform pathways for the fiscal and tax system to empower the green and high-quality development of river basins during the “15th Five-Year Plan” period across four dimensions: promoting policy coupling, stakeholder coupling, spatial coupling, and cycle coupling.