Objective This retrospective study aimed to investigate the correlation between the cardiopulmonary exercise test (CPET) and the severity of coronary stenosis (SYNTAX score) in patients with stable coronary artery disease (SCAD), thereby assessing CPET’s diagnostic value. Methods We classified 220 patients who underwent coronary angiography into coronary heart disease (CHD) and non-CHD groups based on stenosis severity. Patients with SCAD were categorized into mild, moderate, and severe groups by the SYNTAX score. CPET was performed, and data on the general information, serological indicators, and echocardiographic parameters were collected. CPET parameters included oxygen uptake related to the work rate (△VO 2 /△WR), peak metabolic equivalent (Peak Mets), anaerobic threshold (AT), peak oxygen consumption per kilogram (PVO 2 /kg), and metabolic equivalent at anaerobic threshold (Met@AT). Results Patients without SCAD demonstrated significantly higher CPET parameters than those with SCAD. The CPET parameters varied significantly across severity groups in the SCAD group, with the mild group exhibiting higher values than the moderate and severe groups. No significant differences were observed in the baseline data between the groups or among patients with varying numbers of lesion branches. Conclusion CPET parameters demonstrated a significant negative correlation with the SYNTAX score in patients with SCAD, indicating its potential clinical value in assessing the severity of coronary stenosis and diagnosing SCAD.