A low-dispersion single-mode optical fiber includes a core and a cladding covering the core. The core has a relative refractive index difference of 0.30-0.65% and a radius of 2.5-4.5 μm. The cladding layer including first, second, third cladding layers and an outer cladding arranged sequentially from inside to outside. The first cladding layer covers the core, and has a relative refractive index difference of −0.70% to −0.30% and a radius of 4.5-7.5 μm. The second cladding layer covers the first cladding layer, and has a relative refractive index difference of −0.20% to 0.25% and a radius of 7.0-12.0 μm. The third cladding layer covers the second cladding layer, and has a relative refractive index difference of −0.60% to 0.00% and a radius of 10.0-20.0 μm. The outer cladding covers the third cladding layer, and is a layer made of pure silicon dioxide glass.