The ground based arc-scanning synthetic aperture radar (ArcSAR) is a kind of novel ground-based synthetic aperture radar (GBSAR) that has received increasing attention in recent years. The ArcSAR system can cover a wide range of 360-degree scenes by the antenna attached to the rotating boom. Compared with the linear scanning GBSAR, it can effectively overcome the limit of the field of view. Since the ArcSAR system scans along the arc track. The special motion track causes its imaging accuracy is affected by the terrain of the scenes. Traditional ArcSAR imaging algorithms achieve imaging on the reference plane. If the height of the reference plane is inconsistent with the actual height of the target in the scenes, the quality of the imaging result will decrease or even image defocusing will occur. In order to obtain high precision ArcSAR image, we propose a high precision imaging method for ArcSAR based on DEM-assist. We apply the DEM data to assist ArcSAR imaging, which enables the target in the scenes image on its actual height. In this paper, the ArcSAR geometric model and signal model are first introduced. Then the error model of ArcSAR imaging on the reference plane error is analyzed. Besides, the principle of the proposed high precision imaging method for ArcSAR is also given. Finally, the experiments are performed to verify the effectiveness of the proposed method.