Quantum cascade semiconductor lasers (QCL) that emit mid-infrared laser has promising applications in gas molecules detection, especially nitrogen oxides detection. At present, the QCLs have two major forms: pulsed and continuous light. In this study, two nitric oxide detection system using pulsed and continuous light QCLs in the same wavelength were designed. The pulsed light system uses high pulse current to generate chirp modulation; the continuous light system uses sine waves modulation and second harmonic demodulation. The spectrum resolution, measurement range, linearity, limit of detection and other indicators are compared. The results show that the measurement range of the pulsed system is 4.6 times that of the continuous light system, and the detection limit is 2.2 times that of the latter. The continuous system has better linearity. For both systems, the optical noise caused by the Etalon effect is 10 times higher than the electrical noise. So, eliminating Etalon effect will be the focus of the future instrument design.