The working area of F-Theta lens is generally not larger than 420 mm in diameter interiorly when used in laser marking system. F-Theta lenses with larger working areas overseas are of complicated structures. F-Theta lens with its working area as large as 680 mm in diameter is obtained by determining the parameters of initial structure through analysis of the primary aberration and optimizing the initial structure with optical design software. The designed system is compact and composed of only four spherical lenses made up of two kinds of common optical glasses. Its tube length is less than 100 mm. But its focusing performance is diffraction-limited and its distortion relative to the F-Theta linear relation is small. The system satisfies the requirements of marking on a large area.