A spotsize converter (200, 300, 1401), wherein the spotsize converter comprises: a substrate layer; a first coverage layer disposed above the substrate layer, wherein the first coverage layer uses a first material; an isolating layer disposed between the substrate layer and the first coverage layer, wherein the isolating layer uses a second material; and a waveguide (204, 204') disposed inside the first coverage layer, wherein the waveguide (204, 204') is symmetrical along a principal axis, and the waveguide (204, 204') uses a third material; wherein the waveguide (204, 204') comprises an equal-width first waveguide, an equal-width second waveguide, and a third waveguide, a first end of the third waveguide, is connected to a first end of the first waveguide, the first end of the third waveguide is connected to a first end of the second waveguide, and the first waveguide, the second waveguide, and the third waveguide form a Y shape; along a first direction of the principal axis, a width of the third waveguide gradually increases from the first end of the third waveguide to a second end of the third waveguide, and a distance, in a second direction, between the first waveguide and the second waveguide gradually decreases from a second end of the first waveguide to the first end of the first waveguide, wherein the second direction is located on a plane on which the waveguide (204, 204') is located, and the second direction is perpendicular to the first direction; and an absolute value of a difference between a refractive index of the second material and a refractive index of the first material is less than a first threshold, and a difference between a refractive index of the third material and the refractive index of the first material is greater than a second threshold. a substrate layer; a first coverage layer disposed above the substrate layer, wherein the first coverage layer uses a first material; an isolating layer disposed between the substrate layer and the first coverage layer, wherein the isolating layer uses a second material; and a waveguide (204, 204') disposed inside the first coverage layer, wherein the waveguide (204, 204') is symmetrical along a principal axis, and the waveguide (204, 204') uses a third material; wherein the waveguide (204, 204') comprises an equal-width first waveguide, an equal-width second waveguide, and a third waveguide, a first end of the third waveguide, is connected to a first end of the first waveguide, the first end of the third waveguide is connected to a first end of the second waveguide, and the first waveguide, the second waveguide, and the third waveguide form a Y shape; along a first direction of the principal axis, a width of the third waveguide gradually increases from the first end of the third waveguide to a second end of the third waveguide, and a distance, in a second direction, between the first waveguide and the second waveguide gradually decreases from a second end of the first waveguide to the first end of the first waveguide, wherein the second direction is located on a plane on which the waveguide (204, 204') is located, and the second direction is perpendicular to the first direction; and an absolute value of a difference between a refractive index of the second material and a refractive index of the first material is less than a first threshold, and a difference between a refractive index of the third material and the refractive index of the first material is greater than a second threshold.