The imaging performance of high resolution airborne SAR system can be affected by the phase errors resulted from random refractivity fluctuations, when there is a turbulent region in the troposphere. We analyze how the tropospheric refractivity fluctuations affect the high resolution airborne SAR performance. A detailed simulation method is presented to estimate the phase errors caused by atmospheric turbulence. Using the Von Karman model to estimate the refractivity fluctuations in a turbulent region, the structure constant model of atmospheric turbulence and the raw data collected by a Ku-band airborne spotlight-mode SAR with 0.1m resolution, we simulate the actual results of effects of atmospheric turbulence on the SAR image focusing.