In this paper, the basic formula for a double galvanometer deflection system with a distortion lens is given. Various distortion equations and distortion error formulas, and the adjusting method and adjusting error formula are described. The characteristic relationships regarding the constant ratio and the constant error-difference for the system have been deduced. These relationships are consistent with experimental results obtained from large screen display. Finally, a linear scanning scheme with distortion—free on a plane soreen is discussed.