An apparatus for online measurement of a battery internal resistance and a measurement method thereof are disclosed and applicable to online measure the internal resistance of a monomer cell in the battery, which include an excitation source circuit, electrically connected between any two monomer cells to be detected and a battery end and forming a loop respectively or simultaneously with the two monomer cells; an analog-to-digital conversion unit, for detecting and converting a response voltage of the monomer cell ends, and simultaneously electrically connected to the loop formed with the two monomer cells, for detecting and converting a current in the loop as well as detecting and converting a phase value between the current and the response voltage; a digital signal processing unit, electrically connected to the analog-to-digital conversion unit, for calculating the internal resistance value of the monomer cell to be detected after receiving a conversion signal from the analog-to-digital conversion unit; and realize precise, efficient and convenient internal resistance measurement of the monomer cell without damaging the monomer cell and sacrificing the online load of the battery.