Methane (CH4) is one of the most important gases to be monitored in many fields such as atmospheric environment monitoring, industrial process control and coal mine production safety. Developing CH4 sensor has a wide application value. Based on the fundamental frequency absorption band of CH4 molecule near 3.31 μm and its absorption line at 3038.5 cm-1 as the target line, a CH4 sensor based on the mid-infrared room-temperature, continuous and single-mode interband cascaded laser (ICL) is developed. The sensor uses tunable laser direct absorption spectroscopy and long-path (54.6 m) absorption spectroscopy to measure CH4 gas concentrations. A high sensitivity, low power laser temperature controller and current driver are developed independently, and the program of data generation, acquisition and processing based on LabVIEW is coded. The CH4 standard gas with a volume fraction of 2.1×10-6 and a gas dilution system are used to prepare CH4 gas samples with different concentrations. And the performance test of the sensor is conducted. According to the analysis of the standard deviation of the sensor, when the sampling period is 2.5 s, the lower 1σ detection limit of the sensor is about 1.1×10-8. The sensor is used to monitor the concentration of CH4 in the outdoor air for 84 h continuously. The results confirm the engineering practical value of the developed CH4 sensor.