A structure of a direct current power supply circuit in an LED street lamp, wherein the power supply circuit comprises four lines, which are line A， line B， line C and line D, and the voltages on the lines are set so that line A and line B have negative voltages, line C has a positive voltage line D is 0V, and the absolute values of voltages on line A, line B and line C are equal; and line A, line B and line C form a loop with line D via the LED street lamp. Also provided is a method for reforming a street lamp power supply circuit, comprising the following steps: cutting off the power supply circuit from an alternating current grid; providing a switching power supply; connecting the input terminal of the switching power supply to the alternating current grid, and connecting the output terminal of the switching power supply to the original power supply circuit; and connecting the load of the LED street lamp.
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