Background and Aims Currently, the commonly used endoscopic thyroidectomy approaches include the axillary approach, transthoracic breast approach, and transoral approach. Compared with other approaches, the axillary approach creates a natural space between neck muscles. It exposes the thyroid gland in the deep surface of the anterior cervical banded muscle to operate, which minimizes the impact on neck function and avoids the need for CO₂ insufflation, thus reducing the cardiovascular and cerebrovascular effects. Therefore, the axillary approach has been increasingly accepted by surgeons in recent years. However, finding the sternocleidomastoid muscle space is a major challenge in the axillary approach to endoscopic thyroidectomy. Many beginners cannot accurately locate the muscle space in this step, leading to increased surgical time and trauma. In response, our center has made certain improvements to gasless axillary endoscopic thyroidectomy to reduce difficulty finding muscle space during surgery. This study evaluated this modified approach's short-term efficacy and safety to provide a basis for its clinical application.Methods The clinical data of 46 patients with thyroid cancer treated in the Department of Thyroid and Breast Surgery, Suqian First People's Hospital, from January 2023 to May 2023, were retrospectively analyzed. Among them, 23 patients underwent improved gasless endoscopic thyroidectomy via the axillary approach (observation group), while the other 23 patients underwent conventional gasless endoscopic thyroidectomy via the axillary approach (control group). In patients in the observation group before surgery, saline was injected into the space between the sternal and clavicular attachments of the sternocleidomastoid muscle under ultrasound guidance to separate and expand the muscle space. Then, the posterior edge of the sternocleidomastoid muscle at the sternal part was sutured for positioning. After accurate entry into the muscle space, the operation was performed according to the conventional axillary approach endoscopic thyroidectomy method.Results The two groups had no significant differences in general data (all P>0.05), indicating comparability. The average operative time in the observation group was significantly shorter than that in the control group (65.6 min vs. 87.2 min, P<0.05). At the same time, there were no significant differences in intraoperative blood loss, postoperative drainage volume, number of central lymph nodes removed, and length of hospital stay between the two groups (all P>0.05). One patient in the observation group developed a subcutaneous hematoma in the axilla, which was resolved by aspiration and compression. There were no complications, such as difficulty breathing, hoarseness, numbness in the limbs, or coughing while drinking in the remaining patients. Three months after surgery, the two groups had no significant differences in neck pain score, neck injury index, and swallowing disorder index (all P>0.05). After surgery, all patients received individualized thyroid-stimulating hormone (TSH) suppression therapy with oral levothyroxine sodium tablets. During the follow-up period, none of the patients experienced recurrence or metastasis.Conclusion Preoperative ultrasound-guided suture positioning of the sternocleidomastoid muscle combined with saline injection to separate the muscle space is convenient and practical, facilitating the identification of the muscle space during surgery. This approach reduces the surgical difficulty of gasless endoscopic thyroidectomy via the axillary approach and has good clinical application value.