The Terahertz circular Synthetic Aperture Radar(SAR) which combines the terahertz wave and the circular SAR can obtain the higher imaging resolution compared to the traditional linear SAR,and it has a broad application prospect in the radar field. An important factor which influences the imaging quality of radar is the motion error in the real movement which will cause the bad focusing effect, high side lobe and decline in resolution. In the imaging process of terahertz circular SAR,the imaging quality affected by the motion error is determined by the motion errors in different directions together. In order to analyze the accuracy of motion compensation,this paper makes the model of motion error in the terahertz circular SAR and the quantitative analysis of target’s peak descent coefficient of target when the motion error exists. The changes of imaging qualities in cluded resolution,Peak Sidelobe Ratio(PSLR) and Integrated Sidelobe Ratio(ISLR) are quantitativly analyzed by the descent coefficient. Besides,this paper makes some simulations to prove the correctness of analysis. The analysis shows important guiding significance for the imaging of terahertz circular SAR.