An indicating circuit for a switching power supply is provided. An LED and a resistor are connected in series, and then are connected in parallel with a capacitor, and the parallel-connected circuit and a diode are connected in series in the same direction, and then are connected in parallel with an inductor to form the indicating circuit. The cathode of the indicating circuit is a terminal 1, and the anode of the indicating circuit is a terminal 2. A charging loop between a rectifying bridge and an electrolytic capacitor for filtering in a fly-back switching power supply is broken to insert the indicating circuit; when the electrolytic capacitor for filtering is normal, an excitation current of a main power stage basically does not appear in the inductor, and the LED does not emit light; when the ESR of the electrolytic capacitor rises greatly, the excitation current of the main power stage appears in the inductor; furthermore, when a switching transistor in the switching power supply is switched off, the excitation current flowing through the inductor may not be changed abruptly, and is freewheeled by the diode; after being filtered by the capacitor, the excitation current drives the LED to emit light; the LED may be a light emitter of a photocoupler to notify a user that: the ESR of the filter capacitor has risen and the switching power supply has a risk of failure, thereby avoiding aggravation of loss; and the indicating circuit has the characteristics of low cost and easiness in implementation.