A transformer and a switch-mode power supply are provided. The transformer includes: a magnetic core structure; several windings that surround a same magnetic cylinder in the magnetic core structure in a stacked manner, where the several windings include at least one primary-side winding and at least one secondary-side winding; and an electromagnetic shielding layer that is disposed between at least two adjacent windings, where the two adjacent windings are a primary-side winding and a secondary-side winding, and the electromagnetic shielding layer is made of a magnetic material. The electromagnetic shielding layer of the transformer can suppress a noise current of the winding, to reduce noise.