The invention relates to an automatic equalization system device of a power battery, being characterized in that the positive and negative electbares of a battery pack composed of a plurality of serially connected battery cells are respectively connected with slave control unit modules of battery cells; the positive electbare and the negative electbare of each battery cell are connected with corresponding cell equalization unit module through a line; the adjacent battery balancing unit modules are connected with the same control relay through a line; one port of the control relay is communicated with the slave control unit module through the line, and the other port of the control relay is communicated with the control switch through the line and then communicated with the slave control unit module; the slave control unit module is communicated with the master control unit module through the communication line; and the master control unit module is communicated with the display and theinput device through the line. The automatic equalization system device of a power battery can prevent the vehicle from affecting the whole system when the internal pressure difference of the batterysystem is larger than a preset threshold value due to internal self-reason or external problem during long-time operation, and timely balancing maintenance is carried out on the battery of the problem box.