Objective:To investigate the correlation between cytokeratin 19 fragment(CYFRA21-1), modified ultrasound B-line and connective tissue disease associated with interstitial lung disease (CTD-ILD).Methods:The data of 112 patients with CTD hospitalized in the Department of Rheumatology and Immunology of the Second Hospital of Fujian Medical University from September 2019 to December 2021 were retrospectively collected. Sixty patients in the CTD-ILD group and 52 patients in the connective tissue disease without interstitial lung disease (CTD-noILD) group were included. The t-test and χ2 test were used to compare the demographic characteristics and tumor-associated antigens of the two groups of patients. Modified ultrasound score and HRCT Warrick score were evaluated by Pearson correlation analysis. In addition, the relationship between CYFRA21-1, modified ultrasound score and Warrick score were evaluated, and the diagnostic efficacy of CYFRA21-1 and modified ultrasound of CTD-ILD was evaluated and analyzed by binary logistic regression analysis. Results:Patients in the CTD-ILD group had higher CYFRA21-1 concentrations than the CTD-no-ILD group[5.74(4.25, 9.79) ng/ml vs. 2.79(2.21, 3.23) ng/ml, Z=45.94, P<0.001], patients in the CTD-ILD group had higher modified ultrasound scores than the CTD-no-ILD group [44.5(36.5, 60.0) vs. 5.0 (3.2, 6.8), P<0.001]. Modified ultrasound score was positively correlated with Warrick score ( r=0.93, P<0.001) and CYRFA21-1 was positively correlated with modified ultrasound score ( r=0.39, P=0.042). The sensitivity of CYFRA21-1 in determining CTD-ILD was 81.7% and the specificity was 92.3% [ AUC (95% CI)=0.88(0.81, 0.95), P<0.001], the sensitivity of modified ultrasound B-line to determine CTD-ILD was 96.4% and the specificity was 92.9% [ AUC (95% CI)=0.99 (0.97, 1.00), P<0.001]. History of smoking[ OR(95% CI)=9.26(1.11, 77.12), P=0.040] and elevated CYFRA21-1 concentration[ OR(95% CI)=19.40(4.89, 76.95), P<0.001] were risk factors for CTD-ILD. Conclusion:CYFRA21-1 is expected to be a serum marker indicating concomitant ILD in patients with CTD. Modified ultrasound B-line to determine concomitant ILD in CTD patients has good diagnostic utility and can reflect the severity of pulmonary fibrosis in CTD-ILD patients.