Purpose: Treatment of post-operative pelvic side-wall relapses of cervical cancer has always been a therapeutic challenge for radiation oncologists. Radiation dose boost to recurrent tumor by brachytherapy is necessary, but difficult to achieve. Our treatment center has successfully achieved precise transvaginal insertion of a pelvic side-wall mass, using a metal tube with real-time ultrasound guidance. This study investigates the efficacy and safety of image-guided high-dose-rate (HDR) interstitial brachytherapy (IBT) for patients with post-operative pelvic side-wall relapses. Material and methods: Between 2018 and 2020, 36 post-operative pelvic side-wall relapses of cervical cancer patients receiving external beam radiotherapy (EBRT) combined with HDR-IBT were analyzed retrospectively. Doses per fraction ranged from 6.0 to 7.0 Gy, whereas cumulative equivalent doses in 2 Gy fractions ranged from 80 to 100 Gy. Effects of prognostic factors on local control (LC) and progression-free survival (PFS) were analyzed, and late toxicity data were evaluated. Results: A total of thirty-six patients were included, with a median follow-up of 19.3 months. The tumor response was obtained for all patients, with radiological complete remission in 20 (55.6%) patients. Two-years LC and PFS rates were 72.2% and 47.2%, respectively. Grade II rectal toxicity was observed in 5 patients (13.9%). Multivariate analyses for LC and PFS using proportional regression model were performed, in which shape of exophytic tumor was associated with a significantly better prognosis for both LC and PFS (HR = 0.071, 0.128, p = 0.005, 0.002). Clinical target volume (CTV) D(90)( )remained associated with a significantly better prognosis for PFS (HR = 0.056, p = 0.000). Conclusions: A metal tube placed in the vagina, under the guidance of real-time ultrasound in transvaginal IBT for pelvis masses can be accurately achieved. The shape of exophytic tumor and CTV D-90 were associated with a significantly better prognosis for PFS, and the shape of exophytic tumor was also associated with a better prognosis for LC. Therefore, radiation dose boost using IBT can improve the prognosis of patients with post-operative pelvic side-wall recurrences of cervical cancer.