The deltoid is a large bulky muscle, comprising approximately 20% of the shoulder muscles. Therefore, the function of the deltoid as a stabilizer is thought to be significant. The current authors quantified dynamic glenohumeral stability provided by three heads of the deltoid by a new biomechanical parameter, the dynamic stability index. The dynamic stability index considers not only the force vectors generated by individual shoulder muscle, but also the concavity compression mechanism. The higher the dynamic stability index, the greater the dynamic stability. The deltoid generated significant shear force and compressive force in the position of anterior shoulder instability. The deltoid provided dynamic stability with the arm in the scapular plane and only decreased the stability of the shoulder with the arm in the coronal plane. The mid and posterior heads should be strengthened vigorously in anterior shoulder instability in conservative and operative treatment, because they provide more stability generating higher compressive force and lower shear force than the anterior head. Scapular muscles should be balanced to avoid the vulnerable glenohumeral position where the arm is extended beyond the scapular plane. Anatomic reattachment of the detached labrum onto the glenoid rim in the anterior instability is important to enhance the effect of compressive force component generated by the muscles.