In order to meet the requirement for detecting the methane (CH4) leakage in the daily maintenance of the network of gas pipelines in cities, an inhaled portable CH4 detector based on tunable diode laser absorption spectroscopy (TDLAS) technique has been developed, whose weight is only 1.4 kg and volume is 22 cm×10 cm×10 cm. A distributed feedback laser with a central wavelength of 1654 nm is used as the light source in the instrument and an optical multi-pass cell with a physical basic length of 8.5 cm and an effective optical path of 2.25 m is integrated. The system based on STM32F405 generates a stable modulation laser by controlling laser, and acquires the absorption signal by the signal acquisition and process unit, then a digital lock-in amplifier is used to extract the second harmonic component in the signal to get the concentration of gas. Calibration and stability tests of the detector are carried out. Results show that there is a good linear relationship between the amplitudes of second harmonic signal and gas concentrations. The measurement accuracy of system is ±3.05% and the minimum detectable limit is 0.88 ppm. The precision and portability of the instrument meet the requirements of actual measurement.