An optical fiber preform and the manufacturing method thereof are disclosed. The ratio of the diameter of the optical fiber core rod to the diameter of the core layer is 2.1-2.8.The optical fiber core rod is fitted within a small cladding tube, which is made of fluorine-doped quartz glass. The optical fiber core rod and the small cladding tube are melted and combined together to form an assembled core rod. The ratio of the diameter difference between the assembled core rod and the opitical fiber core rod to the diameter of the core layer is 0.5-2.2.The fluorine content Δ F of the small cladding tube made of fluorine-doped quartz glass is -0.20%to-0.35%,and the hydroxyl content is equal to or less than 500 ppb. A large cladding tube made of quartz glass is fitted outside of the assembled core rod, or a SiO2 glass jacket is deposited directly onto the assembled core rod. The ratio of the effective diameter of the optical fiber preform to the diameter of the assembled core rod is 2.5-5.6.Said optical fiber preform can be used to make bend-resistant single mode fiber with low water peak that meets the standard of ITU-T G.652.D and G657.