Tunable laser diode absorption spectroscopy (TDLAS) is a highly promising gas detection technology. However, systems built with lasers often suffer from issues such as laser aging, significant optical path insertion loss, and the need for manual calibration, leading to detection errors. This article designs a methane detection system based on TDLAS technology to address this issue. The device is based on the Lambert Beer law, selecting a laser with a wavelength of 1653.7nm as the light source. A data acquisition card is used to simultaneously collect and characterize the fundamental and second harmonic signals. By using the fundamental signal to calibrate the second harmonic signal in real time, the accuracy of the detection results is ensured, without the need for manual calibration. Afterwards, a comparative experiment was conducted with a system without self calibration, indicating that this theory has significant advantages in stability and has great prospects in practical applications.