A transformer, and switching power supply. The transformer comprises: a magnetic core structure; several windings surrounding a same magnetic cylinder in the magnetic core structure in a stacked manner, wherein the windings comprise at least one primary winding and at least one secondary winding; and an electromagnetic shielding layer between at least two adjacent windings of said windings, the at least two adjacent windings including a primary winding and a secondary winding. The electromagnetic shielding layer consists of a magnetic material. The electromagnetic shielding layer of the transformer can suppress a noise current of the windings, thus achieving noise reduction.