Accurate measurements of the entropic coefficient of lithium-ion batteries (LIBs) are essential for optimizing the battery thermal management systems, which are usually accomplished with the potentiometric method. However, as this method requires much time, its applications are greatly limited in commercial settings. The present work addresses this issue by proposing a fast and precise method, characteristic of a standard square wave current with a DC offset as input, to determine the continuous entropic coefficient profile of LIBs based on frequency-domain analysis. The measurement error introduced by the DC offset current is analyzed and further eliminated by reversing the current. By establishing a differential isothermal testing platform and accurately measuring the transfer function, the proposed method results in a continuous entropic coefficient profile, which agrees well with the discrete values obtained through the potentiometric method, meanwhile the measurement speed is 150 times faster than that of the potentiometric method. In terms of time and accuracy, the proposed method is advantageous and promising in on-board battery diagnostics applications.