Objective To investigate short-term effectiveness of arthroscopic repair via modified subacromial viewing portal (hereinafter referred to as modified viewing portal) in treatment of LafosseⅠsubscapularis tendon tears. Methods A clinical data of 52 patients with LafosseⅠsubscapularis tendon tears, who underwent the arthroscopic repair via modified viewing portal between October 2020 and November 2022 and met the selective criteria, was retrospectively analyzed. There were 15 males and 37 females with an average age of 63.4 years (range, 41-76 years). Twelve patients had trauma history and the other 40 patients had no obvious inducement. The main clinical symptom was shoulder pain and the hug resistance tests were positive in all patients. The interval between symptom onset and admission ranged from 3 to 26 months (mean, 7.2 months). The shoulder pain and function were evaluated by visual analogue scale (VAS) score, American Shoulder and Elbow Surgeons (ASES) score, and University of California Los Angeles (UCLA) score before operation and at 12 months after operation. The shoulder range of motion (ROM) of forward flexion, abduction, and external rotation and the internal rotation strength were measured before operation and at 3 and 12 months after operation. MRI was performed at 3-6 months after operation to assess the tendon healing and the structural integrity and tension of reattached tendon. Patient’s satisfactions were calculated at last follow-up. Results All incisions healed by first intention, no complication such as incision infection or nerve injury occurred. All patients were followed up 12-37 months (mean, 18.5 months). The VAS, UCLA, and ASES scores at 12 months after operation significantly improved when compared with those before operation (P<0.05). The ROMs of abduction and forward flexion and the internal rotation strength at 3 and 12 months significantly improved when compared with those before operation (P<0.05); and the ROMs at 12 months significantly improved compared to that at 3 months (P<0.05). However, there was no significant difference (P>0.05) in the ROM of external rotation at 3 months compared to that before operation; but the ROM at 12 months significantly improved compared to that before operation and at 3 months after operation (P<0.05). Thirty-one patients underwent MRI at 3-6 months, of which 28 patients possessed intact structural integrity, good tendon tension and tendon healing; 3 patients underwent tendon re-tear. At last follow-up, 41 patients (78.8%) were very satisfied with the effectiveness, 7 were satisfied (13.5%), and 4 were dissatisfied (7.7%). Conclusion Arthroscopic repair via modified viewing portal for Lafosse Ⅰsubscapularis tendon tears, which can achieve the satisfactory visualization and working space, can obtain good short-term effectiveness with low overall re-tear risk.