Objective：The current study aims to quantitatively detect myocardial fibrosis of idiopathic inflammatory myopathy（IIM） by 3.0 T cardiac magnetic resonance（CMR），and to analyze the association between myocardial fibrosis of IIM and cardiac injury marker chitinase-3-like protein 1（YKL-40），to explore the value of serum YKL-40 in reflecting the degree of myocardial fibrosis in IIM patients with myocardial injury. Methods：A total of 64 IIM patients who visited the First Affiliated Hospital of Nanjing Medical University between May 2018 and August 2022 were retrospectively included. All patients underwent 3.0T CMR scan and the expression level of serum YKL -40 was measured. We used Student’s 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 association between serum YKL-40 and CMR parameters in IIM patients. Receiver-operating characteristic（ROC）curve was used to assess the value of YKL-40 in predicting myocardial fibrosis 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 expression level［73.4（46.9，119.3）ng/mL vs. 48.1（22.5，65.1）ng/mL］，native T1［（1 263.2 ± 50.3）ms vs. （1221.8 ± 59.3）ms］，and extracellular volume（ECV）［（31.7 ± 5.6）% vs.（28.7 ± 3.2）%］，but 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. YKL -40> 68ng/mL（AUC=0.71，95% CI 0.58~0.84）predicted myocardial fibrosis in IIM with a sensitivity of 0.62 and specificity of 0.86. Conclusion：Serum YKL-40 may reflect the degree of myocardial fibrosis in IIM patients with myocardial injury.