According to the fundamental absorption properties of Carbon Monoxide(CO) near 4.7 μm, a novel CO sensor was designed using a Quantum Cascaded Laser (QCL) whose central wavelength is 4.75 μm and Multi-pass Gas Cell (MGC). This sensor uses a QCL with the thermoelectrically cooled function and can work under pulse mode and room temperature, the exiting optical wavelength was located in a strong absorption line (2 103 cm-1）which is in the base band of CO through adjusting the injection current and temperature. Meanwhile, a novel MGC (40 cm long and 800 mL sampling volume) with 16 meters effective optical path length and mercury cadmium telluride mid-infrared detectors was used, thus effectively improved the sensitivity of this system. Additionally, a reference gas cell and a spatial filtering optical structure were occupied, resulting in effective improvement of the beam quality and reduction of the noise caused by the instability of QCL, the sensitivity of this system was improved furtherly. It indicated that the system works stably by means of multiple measurements to the carbon monoxide gas with different concentration, a detection limit of 5 μmol·mol-1 can be obtained when the signal-to-noise ratio equals 1.