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To analyze the dosimetric effects of shoulder position shift on target volumes and organs at risk (OARs) during radiotherapy for head and neck cancer, 14 patients with different types of head and neck cancer were selected. Based on original positioning CTs (o-CTs), the shoulder position was moved toward the superior inferior (SI) direction, anterior posterior (AP) direction, and the left right (LR) direction by 3 mm, 6 mm, and 15 mm, then 252 new CTs (n-CTs) were produced. After setting the movement, the new adipose tissue density was set to 1 g/cm3, and the movement of the bone tissue was also considered. The original plan (o-plan) on the o-CT was copied to the n-CTs and recalculated to obtain 252 new plans (n-plans). The differences between the o-plan and n-plan target volumes and OARs in field-fixed intensity modulated radiotherapy and the volumetric modulated arc intensity (VMAT) were studied, and the dose changes of the target volume D95% and spinal cord Dmax were compared with the movement of the shoulder position. It was proved that the effects of the shoulder position shift on the target volume and OARs for head and neck cancers are substantial, especially a 15 mm shift toward the superior direction had the greatest influence, reaching 4.16% (VMAT). The closer the target area is to the shoulder, the more serious the impact. The spinal cord dose increased greatly, reaching a maximum of 72.8 cGy, 100 cGy and 212 cGy (VMAT), respectively, for shift of 3 mm, 6 mm, and 15 mm towards the inferior direction. The o-plan and p-plan were analyzed using a 3D γ passing rate (3%/3 mm), that was more than 95% except for the 15 mm shift towards the superior direction. Radiotherapy technicians should pay attention to ensure that the shoulder is lightly touching the thermoplastic mold during radiotherapy positioning for head and neck tumors to prevent patients from shrugging their shoulders. The OAR dose limit should be considered in the planning design, and a sufficient margin should be left to prevent the excessive dose caused by the positioning.