The invention relates to the technical field of battery balancing, and discloses a balancing method of a modularized battery pack system. The voltages VBAT1, VBAT2...VBATN at the two ends of each of a plurality of battery packs are detected by adopting a battery management system (BMS), and whether the dropout voltage between any two voltages is zero or not is judged; if yes, the BMS controls first relays which are connected with the battery packs in series correspondingly to be closed, and electrifying is completed; and if not, second relays connected with the first relays in parallel are arranged, according to the low-to-high sequence of the voltages of the battery packs, the BMS sequentially sends instructions to control all the second relays to be closed and adjusts the resistance of loops where the second relays and the battery packs are located, voltage balancing among the battery packs is achieved, then the BMS sends a switch-off instruction to the second relays and then sends a closing instruction to the first relays, and electrifying is completed. Automatic balancing of the battery pack system formed by the modularized battery packs is achieved. The invention further discloses a balancing system of the modularized battery pack system.