Aiming at the abuse of gaseous substances such as nitrous oxide (N2O), the detection method based on the combination of quantum cascade laser and wavelength modulation technology can detect the concentration of nitrous oxide exhumed by users, and realize the rapid on-site identification of users. Aiming at the measurement error caused by laser light intensity jitter, a digital phase-locked amplifier for trace N2O gas detection is proposed. The laser light stress modulator is integrated in the phase-locked amplifier. When the mercury cadmium telluride detector receives the absorbed light intensity signal, the first and second harmonics are demodulated in FPGA at the same time, and the second harmonics are normalized to avoid the influence of light intensity jitter on the measurement. The performance of the detection system is evaluated by using the phase-locked amplifier. The mass flow controller is used to set different concentrations of N2O gas. When the laser modulation frequency is 5 kHz, the linear fitting degree between the normalized second harmonic peak value and the N2O gas concentration is 0.992 1. The detection system detection limit is evaluated using Allan variance. When the system integration time is 0.1 s, the detection limit of N2O gas is 17.7×10-9; when the integration time is 6.3 s, the detection limit of N2O gas reached its lowest value of 8.9×10-9. The phase-locked amplifier has the advantages of high integration, fast detection speed and high signal-to-noise ratio, and is suitable for the field rapid detection of trace N2O gas.