The present invention discloses a parallel operation expansion power supply system and method. The system is connected with a load, a battery configured to supply power for the load and a busbar of an original power system through the busbar. The power system comprises: a monitoring device configured to monitor the busbar voltage of the busbar and determine that the original power system is a charging mode of charging of the battery according to the busbar voltage; and a charging device configured to charge the battery according to the charging mode. Positive and negative busbars of a new power supply and an old power supply are connected, the parallel operation expansion power supply system and method realize floating charge or equalized charging of a new power supply with the old power supply through new power supply monitoring software process to allow the parallel operation expansion to be carried out on power systems of different manufacturers so as to greatly extend the expansion range of an old power system and have a huge market value.