In order to efficiently extract the first and second harmonic signals from the differential signal in infrared gas detection based on tunable laser absorption spectroscopy, an orthogonal phase lock-in amplifier was developed. According to the principle of orthogonal lock-in amplifier and harmonic detection, the simulative experiment system based on Simulink platform was set up, and its functions were simulated and verified. The hardware system is designed and developed by adopting a digital signal processor as its core controller, which is mainly composed of a 90° phase shifter, two analog multiplier, two low pass filter, a differential signal amplifier and an AD converter. In experiment, firstly, by using standard sine-wave signals with tunable amplitudes as the signals to be measured, the amplitudes of the output signals from the fabricated lock-in amplifier and the standard amplitudes are both tested and compared. Good linear relationship is found between them, the linear fitting degree is as high as 0.999 94, and the maximum error is less than 4%. Secondly, using the simulated differential signal with gas absorption as the signal to be measured, and by using fundamental square wave signal and frequency-doubled square wave signals as reference signals, both first and second harmonic signal are extracted. Because the second harmonic signal is too weak and easy to be polluted by noises, its detection error is less than 5%, and that of the first harmonic signal is less than 3.5%. The proposed system has good applicative prospects in infrared gas detection due to good stability and high price ratio.