The invention provides a circuit for preventing power-off back flash of an LED illuminating device. The circuit relates to the technical field of LED lamp strings, and comprises an AC power supply module, a bridge rectifier combination, an LED lamp string, a constant-current driving circuit used for controlling an LED current, a starting resistor and two filtering voltage-stabilizing capacitors C1 and C2, the filtering voltage-stabilizing capacitors are connected behind the bridge rectifier combination and to a power supply VCC end of the constant-current driving circuit separately, the starting resistor provides an operating current for the constant-current driving circuit, one end of the starting resistor is connected with the power supply end of the constant-current driving circuit and the filtering voltage-stabilizing capacitor C1, and the other end is connected with a backflow-preventing diode D1 which is used for preventing charges on the filtering voltage-stabilizing capacitor C1 from flowing back to the starting resistor. When a power switch is turned off, the starting resistor no longer supplies power to the constant-current driving circuit, the constant-current driving circuit stops operation and does not reboot when the VCC voltage is reduced to below a switching-off voltage of the constant-current driving circuit, and the LED lamp string is no longer turned on even if line voltage VIN is not complete power-off. Therefore, the circuit solves the problem of LED flicker and creates good use experience for users.