Obstructive sleep apnea syndrome (OSAS) is becoming widespread, especially in people with obesity. As it is usually measured by polysomnography (PSG), the role of cardiopulmonary exercise test (CPET), a new exercise capacity test, has not attracted enough attention in OSAS research. In this article, we explore the relationship between CPET results and patients with OSAS. 68 individuals were recruited and divided into three groups: negative/mild group (n = 22, apnea hypopnea index (AHI) < 15), moderate group (n = 22, 15 ≤ AHI < 30) and severe group (n = 24, AHI ≥ 30). Clinical parameters, cardiopulmonary exercise test (CPET) indexes, and apnea hypopnea index (AHI) were compared among the three groups. A multivariate analysis was carried out to assess which factors determine the index of AHI. The ANOVA analyses were used to evaluate the difference among the three groups. Receiver operating characteristic analyses were chosen to detect the prediction efficiency of the CPET index for AHI. The predictive power of using the CPET index (VO₂/kg peak) combined with the body mass index (BMI) of patients in the detection of AHI was significantly better when compared with using the CPET index only. There was a notable correlation between VO₂ and the index of AHI (r = 0.249, P = 0.04). Also, the CPET data (VO₂/kg peak) combined with the BMI of patients has powerful predictive value for the severity of OSAS in patients. It is expected to be a promising way to predict the severity of OSAS in the future. Our study provides a new strategy for predicting whether a patient is in the early phase OSAS.