Synthetic aperture imaging ladar is new active imaging system, which can offer a finer azimuth resolution than that achievable in conventional synthetic aperture radar (SAR) system. The frequency-modulated-continuous-wave (FMCW) signal is analyzed, and the frequency scaling algorithm, which is suitable for airborne synthetic aperture imaging lidar (SAIL) system using FMCW signal in spotlight model, is deduced. And then, simulation of phase screen (PS) distorted by atmospheric turbulence following the von Karman spectrum by using Fourier transform is implemented in order to simulate turbulence. Finally, simulation shows that in the vacuum azimuth preprocessing can remove the ambiguity of the image and the modified algorithm can eliminate the loss of peak power about 8.6～9.3 dB and the defocusing phenomenon, while considering the effect of the atmospheric turbulence, if the synthetic aperture length does not exceed the similar Fried parameter of the atmosphere for SAIL, a well focused image can be obtained.