A compact methane (CH4) detection system is presented and developed by using an alumina ceramic scattering material as its gas cell. Due to the material's high scattering performance, the optical path length of the gas cell at 1653.7 nm can reach 15.96 cm although its physical length along the light transmission direction is only 0.50 cm. The wavelength modulation spectroscopy technique is employed to enhance the detection sensitivity, and the second harmonic gas absorption signal with low noise is detected and processed. The long-term stability of the system is investigated by the Allan deviation analysis method. Detection limits of 4.5 and 2.6 ppm are achieved at averaging times of 20 s and 200 s, respectively. The dynamic gas exchange performance is also experimentally studied. The experimental results indicate that our system is a good choice for practical applications owing to its small volume, high sensitivity, and stability.