Abstract: Objective: To investigate the effect of 8-week scapular muscle rehabilitation training on shoulder function and subacromial space in patients with subacromial impingement syndrome(SIS), and to provide new evidence for exploring the relationship between biomechanical factors and subacromial space. Methods: 48 patients with SIS were randomly divided into exercise group(n=24) and control group(n=24). The exercise group conducted scapular muscle training(three times perweek) for 8 weeksand with two health education courses, while the control group had two health education courses only. The pain, shoulder function and subacromial space of the two groups were evaluated before and after the intervention, the measurements included visual analogue score(VAS), shoulder pain and disability index(SPADI), shoulder joint range of motion, acromio humeral distance(AHD), etc.Results: There were 2 cases dropped in exercise group and 1 case dropped in control group. After 8 weeks of intervention, the VAS scoreand SPADI score were decreased in the exercise group,but the muscle strength of the middle and lower trapezius muscle, the length index of pectoralis minor muscle and the AHD at 60° of shoulder abduction were increased compared to those before intervention(P<0.05). In the control group, the shoulder abduction range of motion was increased significantly(P<0.05), but no significant changes were found in other indicators(P>0.05).Compared with the control group,the strength of the middle and lower trapezius muscle, the length index of pectoralis minor muscle and the AHD at 60° of shoulder abduction in the exercise group were significantly higher(P<0.05). There was a significant positive correlation between the improvement of shoulder function and the increase of subacromial space at 60° abduction(r=0.495, P=0.091). Conclusions: Scapular muscle rehabilitation training can improve the pain and shoulder function in SIS patients, and the increase of subacromial space during shoulder abduction is one of the potential mechanisms.