Objective：To quantitatively detect myocardial fibrosis of idiopathic inflammatory myopathy(IIM) by 3.0 T cardiac magnetic resonance(CMR), and to analyse the relationship with cardiac injury marker chitinase 3-like-1 protein (YKL-40), to explore the value of serum YKL-40 in reflecting the degree of myocardial fibrosis in IIM patients with myocardial injury. Methods：64 IIM patients who visited the First Affiliated Hospital of Nanjing Medical University from May 2018 to August 2022 were retrospectively included. All patients underwent 3.0T CMR scan and serum YKL-40 was measured.We used t test, Mann-Whitney U test or chi square test to compare the difference of each index between the myocardial injury group and the non-myocardial injury group; Spearman correlation analysis and linear regression were used to analyze the relationship between serum YKL-40 and CMR parameters in IIM patients. Results：Compared with patients without myocardial injury,IIM patients with myocardial injury had higher age [(53.0 ± 13.0) year vs. (44.6 ± 16.1) year], serum YKL-40 [73.4 (46.9,119.3) ng/mL vs. 48.1 (22.5, 65.1) ng/mL], Native T1 [(1263.2 ± 50.3) ms vs. (1221.8 ± 59.3) ms], and extracellular volume (ECV) [(31.7 ± 5.6)% vs. (28.7 ± 3.2)%], the left ventricular stroke volume index (LVSVI) was lower [(42.3 ± 10.2) mL/m2 vs. (49.0 ± 11.1) mL/m2] (all P<0.05). Serum YKL-40 was positively correlated with ECV (r=0.373, P=0.004). Multiple linear regression analysis showed that serum YKL-40 was independently correlated with ECV. Conclusion：Serum YKL-40 can reflect the degree of myocardial fibrosis in IIM patients with myocardial injury.