A single mode fiber having a core, an inner cladding, a depressed cladding, and an outer cladding composed of pure silica glass. The core is surrounded in sequence with the inner cladding and the depressed cladding. The core has silica glass doped with germanium and fluorine, with a diameter (a) of 8.0-8.8 μm, a relative refractive index difference (Δ1) of 0.35-0.38%, and the contribution of fluoride (ΔF) is −0.09±0.02%. The inner cladding has silica glass doped with germanium and fluorine, with a diameter (b) of 18-21 μm and a relative refractive index difference (Δ2) of 0±0.02%. The depressed cladding has silica glass doped with fluorine, with a diameter (c) of 26-36 μm and a relative refractive index difference (Δ32) at the external interface thereof is between −0.22 and −0.35%, and a relative refractive index difference (Δ31) at the internal interface thereof is between −0.20 and −0.35%, and Δ32≦Δ31. The fiber has a good bending resistance, good mechanical properties, and extended service lifetime, and prevents the additional stresses generated by bending from passing on to the core, thereby reducing attenuation.