Objective: To study the effect of the upper limbs on dosage in Gamma Knife treatment. Methods: The design function of the Luna-260^TM Gamma Knife Radiotherapy Planning System was utilized, using a body phantom to simulate conventional treatment sites. Twenty sampling points were set for irradiation locations. Using five different collimator sizes commonly used in body treatments, treatment plans were designed under conditions with and without upper limbs, and sampling point irradiation time comparison data was collected to calculate and analyze dose error rates. Results: Across the 20 sampling points, the dose error range was from -16.09% to 0 when comparing treatment plans without upper limbs to those executed with upper limbs present, and from 0 to 19.75% in the reverse comparison. With the same prescription dose, location, and collimator size, dose error increased as the irradiation site moved closer to the upper limbs and decreased as the distance increased. Conclusion: In Gamma Knife treatment, the dose error decreases as the irradiation site is further from the upper limbs and increases when closer. Consistency in upper limb positioning is essential during Gamma Knife localization, planning, and execution. Although small, the upper limbs can significantly impact dosage, requiring stringent quality control to ensure the precision of treatment doses, thus safeguarding the effectiveness and safety of patient treatments.