A high-speed phase modulation of down-looking synthetic aperture ladar was proposed. The spatially polarized beam was divided into two coaxial and polarization-orthogonal beams. In the orthogonal direction of travel, the two beams were modulated to sinusoidal phases, whose changing directions were contrary; in the travel direction, these two beams were modulated with different curvatures to a quadratic phase history, which was associated with position of the targets in slow-time axis. This modulation method has the function of automatic jitter compensation in the azimuthal direction, and can effectively prevent the platform jitter effects on imaging.