Inefficient gas transport in the porous cathode is disastrous for the lithium–air battery to achieve a high electrochemical performance. Previous evaluation of the cathode diffusivity relies on indirect calculations based on multiple V–I data obtained over the intact battery system, which inevitably induces evaluation uncertainty and material waste. In this report, an electrochemical device is designed for the out-of-cell diffusivity measurement in the lithium–air battery with micro/nano-sized cathodes. With the measured diffusivity, a few electrochemical parameters including the limiting current density and the concentration polarization associated with the porous cathodes can thus be directly evaluated. The work facilitates the development of highly-efficient cathode materials in the general field of metal–air battery field.