By using Digital Signal Processor (DSP), a digital lock-in amplifier was experimentally demonstrated for extracting 1f and 2f signals in gas detection based on Tunable Diode Laser Absorption Spectroscopy (TDLAS). The orthogonal lock-in amplifying theory was introduced, the algorithm for extracting harmonic signals was designed, and both hardware structure and DSP software were proposed. By using the prepared CH4 samples within the concentration range of 1%~5% and the developed lock-in amplifier, detailed experiments were carried out to derive the device′s performances. As shown by experimental results, the system′s signal-to-noise ratio at the frequency of 2f signal is 34 dB under the CH4 concentration of 5%, which indicates good functions of the lock-in amplifier. The amplitude ratio between 2f and 1f signals is linear to gas concentration. Considering gas preparation and diffusion time, the response time of the detection system is about 96 ~ 98 s. The concentration variation is -92 ppm~+118 ppm for the measured CH4 sample with a concentration of 20000 ppm. The limit of detection obtained from the Allan variance is 29.52 ppm. Compared with analog and commercial lock-in amplifiers, the self-developed device shows great applications in infrared gas detection because of its simple structure, small size, low cost and easy integration.