The defocusing of moving target and the reduction of signal clutter noise ratio (SCNR) caused by long coherent time in high-resolution SAR lead to increasing difficulty of moving target detection. In view of this situation, we propose an SAR/GMTI moving target detection algorithm based on autofocus and variable threshold constant false alarm rate(CFAR) detection. To increase SCNR of moving targets, this algorithm makes use of the fixed relationship between range phase error and azimuth phase error to estimate and compensate two-dimensional phase error of moving target. CFAR detection with different thresholds is also realized to detect moving target and exclude the false alarm, respectively. Experimental results verify the feasibility of this algorithm that it can detect moving target and reduce false alarm ratio sufficiently.