Objective To analyze the clinical value of magnetic resonance imaging (MRI) in the diagnosis oflesion classification of sacroiliac joint (SIJ) in early ankylosing spondylitis. Methods 88patients with early AS in the hospital from April 2017 to June 2018 were chosen in the study. All patients were given CT and MRI examination. The imaging characteristics and the diagnostic value in SIJ lesion classification with the two methods were compared. Results CT examination found that the SIJ space in the patients with early AS was narrow and fuzzy, the joint surface showed destroyed bone in a jagged shape and multiple small cystic lesions, the side of ilium and the middle and lower parts of the joints were involved. MRI showed that there were fuzzy and roughbone joint surface, bone marrow edema and bone cartilagein patients with grade I to II lesions. T1WI, T2WI and short-term inversion recovery sequence (STIR) showed low or equal signals. MRI of patients with grade IIIIV lesions showed obvious inflammatory lesions, bone marrow edema, osteocartilage damage, increased synovial thickness and fat deposition. T1WI and T2WI showed high signal and STIR signal was inhomogenous or incomplete. The detection rates of grade I and II SIJ lesions in AS patients by MRI were higher than CT (P<0.05). There was no significant difference in the detection rate of grade 0, grade III or grade IV lesions between the two examination methods (P>0.05). The rates of bone cystic change under articular surface, joint erosion, articular surface hyperplasia and sclerosis, facet joint lesions at the 5th lumbar segment and the 1st sacral segment and soft tissue swelling detected by MRI were higher thanCT (P<0.05). With pathological results as the standard, the correct rate of MRI diagnosis for early SIJ lesions (grade 0-II) was higher than that of CT while there was no significant difference in the correct rate between CT and MRI in terms of the diagnosis of advanced SIJ lesions (grade III to IV) (P>0.05). Conclusion MRI is of greatclinical value in the classification of SIJ lesions in patients with early AS. It can be used as the first choice to evaluate the changes of sacroiliac joint, the spine and its accessory structures.