The analysis of exhaled gas is a new technology for measuring the composition and content of exhaled gas, which has wide application in non-invasive detection and analysis of human health. In this paper, we build a tunable semiconductor absorption spectroscopy (TDLAS) gas-analysis system using a low-cost vertical-cavity surface-emitting laser (VCSEL) to realize online measurements of human-exhaled end-tidal carbon dioxide (CO2). The system mainly comprises a laser diode, drive control circuit, photodetector, amplifier circuit, data acquisition card, control software, lock-in amplifier, and Herriot gas cell. The gas cell has a volume of 400 mL and an effective optical path of 20 m, and the laser source has a center wavelength of 1579.57 nm. The system uses the second-harmonic amplitude in wavelength-modulated absorption spectroscopy to determine the concentration of CO2 exhaled by a human body. It achieves a fluctuation range of less than ±0.06% and a sensitivity of 0.14%, which satisfies the detection requirements for human-exhaled CO2, and it can accurately, non-destructively, and efficiently measure human-exhaled gas online. This study provides a new strategy for non-invasive detection of human-exhaled gas and related diseases by near-infrared TDLAS technology.