To observe the expression of salivary liquefied sugar chain antigen (KL-6) in serum of patients with idiopathic pulmonary fibrosis (IPF), and analyze the relationship between KL-6 and the severity of IPF. Methods 80 patients with IPF from March 2017 to March 2021 were selected as the observation group, and 80 healthy people in the physical examination center were collected as the healthy control group. The expression levels of KL-6 protein in serum and BALF were detected by enzyme-linked immunosorbent assay (ELISA). Lung function and high-resolution lung CT (HRCT) were performed in both groups. The expression level of KL-6 and lung function indexes-forced vital capacity (FVC) and forced expiratory volume in one second / predict value were analyzed, FEV1% pred) and the percentage of carbon monoxide dispersion in the predicted value (DLCO% pred) and their correlation with HRCT results. The expression of KL-6 in serum of patients with stable stage and acute exacerbation of IPF was detected by ELISA. ROC curve evaluated the clinical effect of KL-6 in predicting the occurrence of idiopathic pulmonary fibrosis. ResultsThe serum expression levels of KL-6 in the observation group were higher than those in the healthy control group (P＜0.05). The indexes of lung function FVC, FEV1% pred and DLCO% pred in the healthy control group were lower than those in the healthy control group (P＜0.05). HRCT score was higher than that in healthy group (P＜0.05). The KL-6 levels of serum and BALF in patients with HRCT score ＞ 4 were significantly higher than those in patients with HRCT score ≤ 4 (P＜0.05). The expression level of KL-6 protein in the observation group was negatively correlated with FVC, FEV1% pred and DLCO% pred (P＜0.05), and positively correlated with HRCT score (P＜0.05). ROC curve analysis showed that the AUC values of serum KL-6 and lung function DLCO% PRED for predicting IPF were 0.8037 and 0.9616, respectively. Conclusion The expression levels of KL-6 has a clear correlation with pulmonary function parameters, HRCT score and severity of IPF, which can be used as a biological index for the evaluation of IPF condition and prognosis.