Objective To investigate the value of cathepsin S in bronchoalveolar lavage fluid (BALF) in comprehensive assessment of the patients with chronic obstructive pulmonary disease (COPD). Methods A total of 46 patients with stable COPD were included as the study (COPD) group, who paid follow-up visits at the Department of Respiratory Medicine of our Hospital from April 2014 to April 2017. They were divided, based on COPD assessment test (CAT) results, into group A(n=11), B(n=7), C(n=13), and D(n=15). Group A was assigned as CAT scored <10, the ratio of forced expiratory volume in the first second (FEV1) to the expected value ≥50%, and the frequency of acute exacerbations during last year less than twice. Group B as CAT scored ≥10, the ratio ≥50%, and the frequency less than twice. Group C as CAT scored <10, the ratio <50%, or the frequency more than twice. Group D as CAT scored ≥10, the ratio < 50%, or the frequency more than twice. As the control group, 29 healthy volunteers were taken from the same frame with normal lung function. The levels of cathepsin S in BALF were determined with enzyme-linked immunosorbent assay (ELISA). High-resolution CT (HRCT) was used to measure the percentage ratio of low-attenuation area (LAA) to total lung area (LAA%). Results BALF cathepsin S levels in the COPD group were significantly higher than that of the control group. BALF cathepsin S levels related to the severity of the disease as group D > group C > group B > group A (P<0.01) and to the degree of emphysema as LAA grade 3 > LAA grade 2 > LAA grade 1 > LAA grade 0 (P<0.01). The results of univariate analysis showed that LAA classification significantly affected the level of cathepsin S (F＝5.141,P<0.001). Correlation analysis indicated that BALF cathepsin S level in COPD patients correlated positively to the frequency of acute exacerbation in the previous year (r=0.142, P=0.003), CAT score (r=0.309, P=0.017) and LAA% (r=0.497, P<0.001). Conclusion The BALF cathepsin S level reflects the severity of COPD and can be used as a biomarker in the assessment and treatment of COPD.