End-tidal carbon dioxide (EtCO?) is a key physiological parameter for evaluating respiratory, circulatory, and metabolic functions. To address the limitations of existing infrared spectroscopy methods in portable real?time monitoring—such as large size, high power consumption, and poor moisture resistance—this study developed a highly sensitive, miniaturized real?time EtCO? monitoring system based on quartz?enhanced photoacoustic spectroscopy (QEPAS). The system employs a distributed feedback laser with a center wavelength of 2.004?μm to detect the CO? absorption line at 4989.967?cm?1, which is less affected by water vapor interference, and optimizes parameters such as modulation depth (11?mA). Performance tests showed that the system exhibits excellent linear response to CO? concentration (R2?=?0.999), with a detection limit of 49.46?ppm. Volunteer experiments demonstrated that the system can continuously and dynamically monitor exhaled CO? concentration during rest, moderate?intensity exercise, and recovery phases: the concentration remained stable at approximately 3.7% at rest; increased to about 4.8% during exercise due to enhanced metabolism and ventilation; and showed a gradual decline after exercise ceased. This study verifies the high sensitivity and stability of QEPAS technology for real?time monitoring of exhaled CO? during exercise, providing a technical basis for the development of portable devices aimed at individualized exercise health management and smart healthcare.