Provided is a power supply circuit for a modified-Siemens-process polycrystalline silicon production device. The power supply circuit comprises a plurality of reducing furnaces, a power supply system formed by N starting power supplies, and K standby starting power supplies. The N starting power supplies and the K standby starting power supplies are connected with a three-phase power transmission circuit at the same time. An output terminal of the power supply system is connected with the plurality of reducing furnaces. Each of the K standby starting power supplies is connected with the output terminal of the power supply system via a phase selector. When any of the N starting power supplies fails, the output phase of any of the standby starting power supplies is adjusted via the phase selector so as to be matched with the output phase of the failed starting power supply which is therefore replaced. In the invention, through the arrangement of the standby starting power supplies, the standby power supplies are used to start the reducing furnaces when the primary power supplies fail, thereby improving the device running stability and increasing the production efficiency and the production capacity as well.