The fractional exhaled nitric oxide (FeNO) is closely related to lung and respiratory diseases. Authorities such as the US Food and Drug Administration (FDA), European Respiratory Society (ERS), and American Thoracic Society (ATS) have successively identified FeNO as an asthma biomarker and listed FeNO as a routine inspection item for respiratory diseases. In order to meet the needs of high-sensitivity monitoring of FeNO, a mid-infrared wavelength modulation laser absorption spectrometer is developed in this paper. The problem of cross interference of CO₂ spectral lines is solved by the method of multiple linear regression, and the detection limit of NO reaches 0.12×10-9. The performance of the prototype is calibrated by standard gases with different concentrations, and the linear correlation of the instrument results is 0.994 in the volume fraction range of 0-215×10-9. Finally, the breath samples of volunteers from Xiyuan Hospital of Chinese Academy of Chinese Medical Sciences are measured.