Extremely straight interference fringes can be formed at the position far away from the waist of Gaussian beams and based on this principle, a novel far-field-interference-based scanning beam interference lithography (SBIL) optical system is proposed. The analytic expression of the nonlinear error with respect to waist radius of Gaussian beams, the incident angle and the waist-to-substrate distance is established. By the numerical simulation, the relationships between the nonlinear error of fringes and the above parameters are analyzed in detail. The research results show that this optical system can effectively limit the nonlinear error of fringe phase to the nanometer scale, and possesses the advantages of simple optical path and high tolerance of assembly errors. The problem that the nonlinear error of fringes at the boundary of the exposure spot is deteriorated can be effectively solved if the waist-to-substrate distance is suitably shortened.