The application discloses an energy storage system and a control method and device thereof. The method includes: sending (Pmax-PV)*Pi/P[Sigma] and PFV to a control unit in an i-th normally operating energy storage unit,wherein i=1, 2, ..., N, N is the total number of the energy storage units in normal operation, PV is the sum of active power of all incoming lines, and PFV is the minimum power factor of all incoming lines, Pci and Pi are the current charging power and the rated charging power of the i-th normal operating energy storage unit, P[Sigma]=P1+P2+...+PN, Pmax is the maximum power demand of the energy storage system from a grid; and causing the control unit to control the maximum charging power of the current energy storage unit at a next moment not to exceed the minimum value of the Pci+(Pmax-PV)*Pi/P[Sigma] and the Pi so as to ensure that PV is less than or equal to Pmax, and also causing the control unit to adjust the reactive output of the current energy storage unit so that the PFV is not lower than the lower limit of the power factor.