Regulating local CO2 concentration and pH near the catalytic active sites could induce effective reaction pathways for electroreduction of CO2 to chemicals. Here, a membrane nanofluid reactor for efficient electroreduction of CO2 to formic acid was prepared by constructing high-density active sites of 3D-Ag nanosheet array on the surface and within the pores of carbon membrane (3D-Ag NA/CM). The local microenvironment of CO2 and H+ concentration was controlled by adjusting the pore size, CO2 flow rate and mass transfer mode. In the flow-through mode, the CO2 was electro-reduced to formic acid with Faraday efficiency of 94% and the partial current density of 35.8 mA/cm2. Both the selectivity and activity were higher than those in flow-by mode. The low local concentration of CO2 in the channel was the key to the high selectivity of formic acid. When the CO2 flow through the membrane nanopores, the high collision frequency of CO2 with catalyst on the pore walls facilitated the electroreduction of CO2. It found that the presence of OV promoted the adsorption of CO2 and reduced the energy barrier for its activation into *CHOO. It lays a foundation for the research on optimizing the performance of CO2 electroreduction with nanofluid reactors.