The main reasons for the sliding of a bag on the left side of the neck include the following: firstly, it is considered to be caused by a thyroid mass, which typically moves up and down with swallowing, mostly due to thyroid adenoma or thyroid nodular goiter, which can be detected by thyroid ultrasound; secondly, it is considered to be caused by swollen lymph nodes in the neck. There are a large number of lymph nodes in the neck, and there are submandibular and subchin lymph nodes on the left side of the neck. If the swelling of the lymph nodes in the neck is stimulated by the local or systemic inflammation, it will show as a mass on the left side of the neck.