In terms of the diploid relationship between scanners sway angle and light beam sway angle, the transient equation of lag angle is derived on the condition that scanning control function is simple harmonic wave. For parallel beam scanners, scanners sway angle is indirectly linear with the angle of scan field of view (FOV). The larger the telescope enlargement factor, the lower the scan efficiency. And the scanner lag angle is equal to the FOV lag angle. Numerical calculation indicates that when the parallel beam scans at 340 Hz of simple harmonic wave and the range of action is above 1000 m, the lag effect in spatial field is obvious and the heterodyne efficiency of image element is much below the threshold 20%. For convergent beam scanners, scanners sway angle is approximately linear with the FOV of scan system, and the proportional constant is related to the relative positions of the scanners. Scan efficiency of the system can be greatly increased by properly designing the relative positions of the scanners. Numerical calculation indicates that when the convergent beam scans at 340 Hz of simple harmonic wave and the range of act is above 3000 m, its maximum lag angle is 1/19 of that of the parallel beam scanner, so the lag effect is largely reduced.