A highly-accurate atmospheric CH4 sensor system using mid-infrared room-temperature Continuous-Wave (CW) Interband Cascade Laser (ICL) was reported. The emitting wavenumber range of the ICL is from 2 998.4 to 2 999.6 cm-1, which covers a strong CH4 absorption line at 2 999.06 cm-1. In order to enhance CH4 absorption, a multi-pass gas cell with a volume of 220 mL, optical path length of 54.6 m was employed. A LabVIEW platform as well as a Data Acquisition Card (DAQ) was used to generate the scan and modulation signal of the ICL and extract the second harmonic signal from the detector output signal. Experimental results showed that when the averaging sampling time was 3.3 s, the Allan deviation of the sensor system was 11.2 ppbv. This sensor system was used to measure the atmospheric CH4 inside and outside the laboratory for a long time, and it is proved to be of great practical value in engineering.