The intra-pulse spectroscopy based on room-temperature pulsed quantum cascade(QC) lasers was introduced. With a room-temperature pulsed QC laser centered at 1904 cm-1, the sample NO gas has been detected.The least-squared algorithm of baseline fitting and concentration retrieving method were introduced for the single line spectroscopy detection, and the gas concentration can be calculated with scan integration and the corresponding values from HITRAN04 database by direct-absorption technique, without calibration. The detection limit was obtained by analyzing the residual of the absorption spectroscopy as 34 ×10-6m.