Objective To investigate the effect of ultrasound-guided intra-articular injection of shoulder joint combined with hydraulic dilatation in the treatment of frozen shoulder in different clinical stage. Methods The clinical data of patients with frozen shoulder diagnosed in Zigong Fourth People’s Hospital from January 2020 to January 2021 were collected retrospectively. The patients were divided into three groups according to the clinical stage of frozen shoulder (stage Ⅰ, stage Ⅱ and stage Ⅲ). All patients volunteered to receive ultrasound-guided intra-articular injection of shoulder joint combined with hydraulic dilatation for 3 consecutive times, with a 2-week interval for each treatment, and followed up for 6 months. Before treatment and 2 weeks, 4 weeks, 2 months, 4 months and 6 months after treatment, the patients were scored with Visual Analog Scale (VAS) and Active Range of Shoulder Motion (AROM), and the capsule thickness of shoulder were measured, respectively. Results A total of 150 patients were included, including 61 patients in stage Ⅰ, 74 patients in stage Ⅱ and 15 patients in stage Ⅲ. The thickness of shoulder joint capsule on the affected side was thicker than that on the healthy side (t=24.384, P<0.001). After treatment, the thickness of shoulder joint capsule and VAS score decreased (P<0.05), and the AROM score increased (P<0.05). Before treatment, the thickness of shoulder joint capsule in stageⅡpatients was higher than that in stageⅠand Ⅲ patients [(4.3±1.0) vs. (2.9±0.5) vs. (3.1±0.4) mm; F=57.195, P<0.001]. Four weeks, 2 months, 4 months and 6 months after treatment, the decrease of shoulder joint capsule thickness was the largest in stageⅡpatients (P<0.05). Compared with stageⅡand Ⅲ patients, stageⅠpatients had the highest VAS score before treatment (7.7±0.7 vs. 5.1±0.8 vs. 4.5±0.7; F=233.560, P<0.001). There was no significant difference in VAS scores among the three groups 4 weeks (F=1.679, P=0.190), 4 months (F=1.348, P=0.263) or 6 months (F= 0.940, P=0.393) after treatment, while there were significant differences in VAS scores among the three groups 2 weeks (F=66.924, P<0.001) and 2 months (F=9.598, P<0.001) after treatment. Compared with stageⅠand Ⅲ patients, stageⅡpatients had the lowest AROM score before treatment (15.2±1.8 vs. 23.5±1.9 vs. 26.1±3.5; F=343.718, P<0.001). Four weeks (F=0.034, P=0.967), 2 months (F=0.222, P=0.801), 4 months (F=0.634, P=0.532) or 6 months (F=0.001, P=0.999) after treatment, there was no significant difference in AROM scores among the three groups, while there was significant difference in AROM scores among the three groups 2 weeks after treatment (F=177.135, P<0.001).Conclusions Ultrasound-guided intra-articular injection of shoulder joint combined with hydraulic dilation has good efficacy in the treatment of frozen shoulder in different clinical stage. Early injection treatment can shorten the course of the disease and relieve the pain of patients during the course of the disease.