A quantum cascade laser (QCL)-based absorption sensor using a novel compact multi-pass gas cell (MGC) for the simultaneous monitoring of CO and N2O is reported. A wide-tunable continuous wave, mode-hopfree external-cavity QCL operating at 4.3 mm is used in the sensor. The QCL can scan over the absorption peaks of N2O (2203.73333 cm-1) and CO (2203.161 cm-1) within a short scan process (1 s), and achieve the simultaneous detection of N2O and CO. The laser is reflected for 243 times, and the cell has an effective optical path length of 29 m with the physical length of 12 cm. Wavelength modulation spectroscopy with second harmonic (2f) detection is performed to achieve the high sensitivity detection of N2O and CO. The detection limit of this sensor is 2.0 × 10-9 for N2O and 1.7×10-9 for CO.