The concentration changes of exhaled carbon dioxide and water vapor are closely related to the physical condition. Therefore, it is of great significance to detect their concentrations. An exhaled gas detection device based on a 2.73 μm distributed feedback laser is proposed. Spectral lines at 3659.402 cm -1 and 3659.934 cm -1 are selected to measure carbon dioxide and water vapor respectively by using the wavelength modulation spectroscopy. The results show that using the second harmonic signal to calibrate the gas concentration, the linearity of 0.99945 and 0.99679 is obtained when the volume fractions of carbon dioxide and water vapor are less than 35% and 2.3%, respectively. The concentrations of carbon dioxide and water vapor during the respiratory cycle are measured in real time. With the measurement time of 0.92 s, the sensor achieves a detection sensitivity of 4.33×10 -3 and 1.37×10 -4 for carbon dioxide and water vapor, respectively. At the acquisition time of 56.8 s, the detection accuracy with 0.12% of carbon dioxide is achieved, and a detection limit of 1.49×10 -4 at the optimal integration time of 17 min is achieved for carbon dioxide measurement.