Arthroscopic release is effective for patients with shoulder stiffness, but the traditional inside-out procedure cannot effectively alleviate the mobility of some severe stiff shoulder and even cause itrogenic injuries sometimes. The aim of this study is to evaluate the clinical efficacy and advantages of a modified outside-in shoulder release approach for severe shoulder stiffness. Included in this retrospective study were 15 patients (five male and 10 female) with severe shoulder stiffness who underwent modified outside-in shoulder release surgery at our hospital between June 2019 and March 2021. Of them, 10 patients had a primary frozen shoulder and five had secondary shoulder stiffness, involving the right shoulder in six cases and the left shoulder in nine cases. The mean age of the 15 patients was 56.7 (34-69) years. The patients were instructed to exercise passively from second-day post-operation and enhance the rehabilitation exercise gradually. All patients received a range of motion (ROM) examination before and after surgery. The American Shoulder and Elbow Surgeon's Score (ASES), Constant Score (CS), and Visual Analog Scale (VAS) score for pain were recorded. All data were tested by normal distribution first and then by paired T test, otherwise by Wilcoxon rank sum test. The mean follow-up period was 18.2 (12-33) months. Compared with the preoperative value, the mean ASES score at the final follow-up improved from 38.4 ± 7.37 to 88.13 ± 6.33 points; the mean CS score from 43.27 ± 6.71 to 78.74 ± 6.93 points; the mean VAS score from 5.07 ± 1.03 to 0.81 ± 0.83 points; forward flexion from 81.93° ± 11.45° to 156.73° ± 9.12°; abduction from 65.93° ± 16.82° to 144.80° ± 8.83°; neutral external rotation from 13.53° ± 10.38° to 51.20° ± 4.77°; internal rotation from the buttock to waist (L3), all showing a significant difference (P< 0.0001). No serious complication was observed in any patient during the postoperative follow-up periods. The present study has demonstrated that the modified arthroscopic outside-in shoulder release approach can improve ROM of patients and alleviate pain effectively, proving it to be an appropriate surgical option for the treatment of severe shoulder stiffness.