Mid-infrared lasers are very suitable for high-sensitive trace-gases detection in that their wavelengths cover the fundamental absorption lines of most gases. Quantum-cascade lasers have been demonstrated to be ideal light sources with their especially high power, wide range of tuning capability and favorable operating condition on roomtemperature. The intra-pulse spectroscopy based on a room-temperature distributed-feedback pulsed QC laser is a simple and effective trace gas detective method to detect trace-gas qualitatively or quantificationally. When a long excitation pulse is applied to a QC laser, the laser frequency tunes almost linearly to lower wave number(lower frequency) as a function of time so all absorption spectral elements are recorded during a single laser pulse. In the present paper, the method was introduced, and identification of N₂O spectral fingerprint using this spectroscopy was demonstrated experimentally. The thermal chirp from a 500 ns long excitation pulse was applied to a quantum-cascade laser to get a fast wavelength scanning, thus a wave number tuning of about 1 cm^-1 was produced. The N₂O absorption spectrum centered at 1273.7cm^-1 was also obtained. The measured absorption spectrum is consistent with HITRAN data precisely.