Objective To study the adverse effect of mirror artifacts generated from the specular reflection of trachea wall on microwave ablation (MWA) treatment of thyroid nodules and the causes. Methods Perioperative ultrasound imaging data of 304 patients who received MWA of thyroid nodules in Shanghai International Medical Center from Jan. 2018 to Oct. 2018 were systemically studied. The occurrence rate, sources, acoustic characteristics of the mirror artifacts and the adverse effects of mirror artifacts on MWA were summarized. The factors contributing to specular reflection of trachea were analyzed. Results A total of 169 cases (55.59%) presented tracheal mirror reflection artifacts in this study. The sources of mirror artifacts included thyroid nodules (20 cases, 11.83%), injection needle (133 cases, 78.70%), ablation electrode (16 cases, 9.47%), hydro-dissection zone (137 cases, 81.07%), and remote ablated area of thyroid nodule (8 cases, 4.73%). We also found that 88.17% (149/169) of mirror artifacts were during the MWA operative period. Conclusion Mirror artifacts generated from the specular reflection of trachea wall are commonly seen in the perioperative period of thyroid nodule MWA. Lesion close to the trachea, flattened tracheal wall, and flat interface linearity larger than the real source dimension are the important causes of mirror artifacts. Direct compression on the tracheal wall caused by thyroid nodules, ablated areas and hydro-dissection zone are the main causes of flattened tracheal wall.