For the gas detection system based on pulse-type infrared quantum cascade lasers, a μs-level narrow-pulse lock-in amplifier was designed and achieved to accurately extract the amplitude of the narrow pulse sensing signal. According to the characteristics of the microsecond narrow pulse signal, the narrow pulse signal passed through a narrow bandpass filter and a fundamental sine-wave signal was obtained. Then, the sine-wave signal passed through a preamplifier, a phase-shifter and a phase-sensitive detection circuit, so that a direct current signal related to the pulse amplitude was obtained. As a testing signal, the narrow pulse whose amplitude, frequency and phase were adjustable was produced by a signal generator, the detailed functional verification experiments were carried out to validate the performance of the proposed lock-in amplifier. The results show that, a good linear relationship is found between the output direct current voltage and the amplitude of the standard input pulse signal; the fitness of the system is about 98.043%; the relative detection error of its output voltage is less than 3%; within one hour′s test, the fluctuation range of the measured signal is less than 1‰. The detailed gas detection experiments were carried out using prepared carbon monoxide (CO) samples with different concentrations and the lock-in amplifier. Within the CO concentration range of 0~180 parts per million (ppm), the output voltage from the lock-in amplifier reveals an exponential relationship with CO concentration, which also proves the normal function of this device. The limit of detection predicted from the Allan variance is 0.412 3 ppm. Compared with commercial lock-in amplifiers, this amplifier has special characteristics including small volume, low cost, ease of integration, and so it has a good application prospect for mid-infrared gas detection based on quantum cascade lasers.