Disclosed is a transformer with a filtering function for a switching power supply. The transformer comprises a magnetic core, a coil framework, a primary winding and a secondary winding, wherein the primary winding and the secondary winding are wound around the coil framework; the coil framework is connected with the magnetic core; the magnetic core is an E-shaped magnetic core; an insulating isolating layer is arranged between the primary winding and the secondary winding; the insulating isolating layer adopts an insulating isolating band; the insulating isolating band is wound between the primary winding and the secondary winding; the insulating isolating layer is prepared from the materials of teflon, or a silica gel band or ceramic; and the switching power supply comprises the transformer and an external circuit, wherein the secondary winding of the transformer is directly connected with the external circuit. A filter is not required to be configured in the switching power supply, so that the output voltage of the transformer for the switching power supply is low in fluctuation, the current can be stably and smoothly output, the circuit is simpler to control, the switching power supply is smaller in size, and the cost is lower.