A single-mode optical fiber, comprising: a core layer; and claddings surrounding the core layer, wherein a relative refractive index difference Δ1 of the core layer ranges from -0.1% to +0.1%, a radius R1 of the core layer ranges from 4.0 μm to 6.0 μm; wherein the claddings surrounding the core layer comprises a first cladding layer, a second cladding layer, and a third cladding layer; wherein the first cladding layer immediately surrounds the core layer, and has a relative refractive index difference Δ2 ranging from -0.2% to -0.6%, and a radius R2 ranging from 10 μm to 22 μm; wherein the second cladding layer immediately surrounds the first cladding layer, and has a relative refractive index difference Δ3 being less than Δ2, and wherein the relative refractive index difference Δ2 and the radius R2 of the first cladding layer and the relative refractive index difference Δ3 and a radius R3 of the second cladding layer satisfy the relationship of V = (Δ2-Δ3)x(R3-R2), wherein a value of V ranges from 0.15% μm to 0.8% μm; wherein the relative refractive index difference Δ3 of the second cladding layer ranges from -0.47% to -0.7%, and the radius R3 of the second cladding layer ranges from 13 μm to 27.5 μm; and wherein the third cladding layer comprises one or more sub-layers (301, 302) immediately surrounding the second cladding layer, and wherein a relative refractive index difference of each sub-layer (301, 302) of the third cladding layer is greater than Δ3; the term "relative refractive index difference" is defined by Δ = / 2 n i 2 n i 2 − n 0 2 × 100 % ≈ n i − n 0 n 0 × 100 %, where ni and n0 are respectively a refractive index of each corresponding portion and a refractive index of pure silica quartz glass. a core layer; and claddings surrounding the core layer, wherein a relative refractive index difference Δ1 of the core layer ranges from -0.1% to +0.1%, a radius R1 of the core layer ranges from 4.0 μm to 6.0 μm; wherein the claddings surrounding the core layer comprises a first cladding layer, a second cladding layer, and a third cladding layer; wherein the first cladding layer immediately surrounds the core layer, and has a relative refractive index difference Δ2 ranging from -0.2% to -0.6%, and a radius R2 ranging from 10 μm to 22 μm; wherein the second cladding layer immediately surrounds the first cladding layer, and has a relative refractive index difference Δ3 being less than Δ2, and wherein the relative refractive index difference Δ2 and the radius R2 of the first cladding layer and the relative refractive index difference Δ3 and a radius R3 of the second cladding layer satisfy the relationship of V = (Δ2-Δ3)x(R3-R2), wherein a value of V ranges from 0.15% μm to 0.8% μm; wherein the relative refractive index difference Δ3 of the second cladding layer ranges from -0.47% to -0.7%, and the radius R3 of the second cladding layer ranges from 13 μm to 27.5 μm; and wherein the third cladding layer comprises one or more sub-layers (301, 302) immediately surrounding the second cladding layer, and wherein a relative refractive index difference of each sub-layer (301, 302) of the third cladding layer is greater than Δ3; the term "relative refractive index difference" is defined by Δ = / 2 n i 2 n i 2 − n 0 2 × 100 % ≈ n i − n 0 n 0 × 100 %, where ni and n0 are respectively a refractive index of each corresponding portion and a refractive index of pure silica quartz glass.