Objective:To investigate the correlations between subacromial impingement syndrome (SIS) and acromial morphology and subacromial intervals after surgery of proximal humerus fracture.Methods:A retrospective study was conducted of the 62 patients with proximal humerus fracture who had been treated by internal fixation with a locking titanium plate from December 2014 to December 2019 at Department of Orthopedics, People's Hospital of Gaoming District. They were 40 men and 22 women, with an average age of 53.5 years (from 35 to 71 years). By the Neer classification, there were 38 three-part and 24 four-part fractures. The incidence of SIS was determined by Nikolaus's diagnostic criteria. The patients were divided into a SIS group and a non-SIS group. The acromial morphology was observed and the subacromial intervals [acromio-greater tuberosity of humerus interval (AGI) and acromio-plate interval (API)] were measured on postoperative X-ray films. The correlations were analyzed between them and SIS.Results:All patients completed treatments and follow-ups from 3 to 15 months (average, 10 months). There was no incision infection or internal fixation failure. There were 18 cases in the SIS group and 44 ones in the non-SIS group. In the SIS group, 3 cases had type Ⅰ acromion while 15 ones type Ⅱ or type Ⅲ acromion; in the non-SIS group, 20 cases had type Ⅰ acromion while 24 ones type Ⅱ or type Ⅲ acromion. The proportion of patients with type Ⅱ or type Ⅲ acromion in the SIS group was significantly higher than that in the non-SIS group ( P<0.05). AGI and API in the SIS group were (11.6±2.1) mm and (14.2±2.4) mm, significantly shorter than those in the non-SIS group [(15.7±2.8) mm and (18.5±3.2) mm] ( P<0.05). Conclusions:SIS is a common complication after surgery of proximal humerus fracture. Abnormal acromial morphology (type Ⅱ or type Ⅲ acromion), poor fracture reduction (reduced AGI), and high placement of a titanium plate (reduced API) may be all important factors leading to SIS.