A power supply method with parallel-connected batteries employs at least two parallel-connected power supply branches, each of which includes a battery, a step-up circuit, and a step-down circuit, and each of which is installed with an individual control unit. The battery offers a low voltage transformed into a high voltage via the stepping-up. The high voltage thence acquires a needed power-supply voltage through the transformation executed by the step-down circuit. An anode of an output of the power supply branch serially connected with a diode prevents the other power supply branch from a reverse current. A cathode of the output serially connected with a current limiting resistance balances the currents on each power supply branch. Accordingly, the present invention diminishes the workload of the device maintenance, lessens users' investment pressure, and provides electricity systems of the power plant and the substation with a consistent direct current power source.