It is important that the light beam is focused into fiber core precisely with a small size when low-order fiber Bragg grating (FBG) is inscribed point-by-point with infrared femtosecond laser. The beam-spot size in fiber core is analyzed theoretically and simulated with Zemax software in the cases of focusing with an objective lens and focusing with an objective lens combined with a cylindrical lens separately when the optical fiber is thought as a cylindrical lens. The beam size is 0.67 μm×84 μm theoretically and 2.6 μm×76 μm simulatively separately when focusing using an objective lens, and the beam size is 3.76 μm×0.67 μm theoretically and 4.1 μm×2.2 μm simulatively separately when focusing using an objective lens combined with a cylindrical lens. It is found that the beam-spot size in the core can become very small when focusing using an objective lens combined with a cylindrical lens. This conclusion will provide possibility and basis for the fabrication of low-order FBG.