An ultrahigh-resolution microwave photonic-based Synthetic Aperture Radar (SAR) imaging method based on space-variant motion error analysis is proposed to solve the influence of space-variant motion error on microwave photonic-based SAR imaging. First, the judgment rules of the influence of space-variant motion error are proposed to analyze the residual space-variant motion error. Second, on the basis of the different judgment results of microwave photonic-based SAR system conditions, the corresponding imaging process is proposed. Finally, the proposed judgment rules and imaging method are verified by point simulation, and the measured data of the 10 GHz microwave photonic-based ultrahigh-resolution SAR are analyzed and imaged. The experimental results show the effectiveness of the proposed method.