The sleeve structure plays a key role in womenswear pattern design, and the proper determination of sleeve angle directly affects the fit and wearing comfort of the finished garment. This paper starts with the two types of sleeve angles in set-in sleeves, discussing respectively the formation mechanisms and constraining factors of the vertical arm-raising angle and the forward-backward inclination angle of the sleeve. By analyzing the corresponding relationship between sleeve cap height and sleeve width, the indirect influence of armhole morphology on sleeve angle, and the inclination angle data of the human arm in its natural hanging state, combined with pattern-making practice, this paper proposes specific control methods and adjustment principles for sleeve angle determination. This paper holds that the determination of sleeve angle requires comprehensive consideration of multiple factors including fabric properties, wearing scenarios, human body structure, and style design. Fixed values alone cannot meet the actual pattern-making requirements, and systematic flexible adjustments should be made on the basis of standard reference values.