Objective In this study，we aimed to verify the methodology and feasibility of fusion imaging of the sacroiliac joints （SIJ） of ankylosing spondylitis （AS） patients. Methods We performed ultrasound（US）-CT and US-MRI fusion imaging of SIJs with 84 inpatients and 30 controls to compare the fusion success rates at 3 cross-sectional levels of the posterior sacral foramina （level1-3，respectively）. Possible factors impacting the fusion success rate were considered and analyzed including the lesion classes of SIJ，the skin fold thickness of the sacral region and the morphological difference of 3 SIJ levels.Results Data showed high success rates of image fusion of US-CT at the 3 levels in both the patient group and the control group.However，no significant differences were observed between the groups at the 3 levels （all P＞0.05）；Success rates of image fusion of US-MRIT1WI and US-MRIT2WI in patient group at the 3 levels were lower than 50%，no significant differences were observed between the two groups at the 3 levels （all P＞0.05）.In the cases of US-CT image fusion，the differences in the imaging success rates between the 4 classes of SIJ lesions were not significant at any level （P=0.136，P=0.708，and P=0.311）.Furthermore，a difference in the mean skinfold thickness at each of the 3 SIJ levels was observed between the successful and failed cases in both the patients’ and control group.Conclusion We established the methodology of fusion imaging of the SIJ.It is suggested that US-CT fusion imaging is feasible with SIJs of AS patients；the skinfold thickness was identified as the main factor affecting the success rates.