The present invention provides a Ton timing circuit, a method and a switching power supply capable of reducing electromagnetic interference of the switching power supply, of which the circuit is a simple and easy-to-implement EMI reduction circuit of the switching power supply that can be integrated in a chip. This circuit adds a variable current source module and a random signal generation module on the basis of the original circuit, the random signal generation module generates N random square waves, which are used for the variable current source module and the constant current source I1 to generate N+1 variable currents, produce different frequencies, eliminate the resonant spikes at the switching frequency, and thus reduce electromagnetic interference caused by switching.