A detecting system of acetylene (C2H2) gas in near-infrared region is developed based on the tunable diode laser absorption spectroscopy. The wavelength modulation spectroscopy in combination of multi-pass absorption cell is used to improve detecting sensitivity of the system. To optimize detecting conditions of the system, spectral signals of standard C2H2 gas with volume fraction of 5×10 -5 are detected under the conditions of pressure of 5.3-12.0 kPa and modulated amplitude of 0.010-0.035 V. The second harmonic signals of C2H2 with different concentrations are detected at the total pressure of 10.7 kPa. To verify stability of the system, spectral signals for 60 s are acquired, and the most suitable detection time as well as the minimum detection limit is obtained by the Allan variance analysis. The results show that the signal intensity of the second harmonic signals is the greatest at pressure of 10.7 kPa and modulation amplitude of 0.030 V. The volume fraction of C2H2 is with a good linear relationship with the amplitude of the second harmonic signals, and the detecting deviation is less than ±2% in the range of volume fraction from 1×10 -6 to 5×10 -5. The most suitable detection time of the experimental system is 7.2 s, and the minimum detection limit is 2.8×10 -11. The instrument adopts a laser driver and a digital lock-in amplifier based on an embedded system, so the instrument has the characteristics of simple mechanical structure, small size, and easy integration with other systems. Therefore, the compact detecting system for C2H2 gas is suitable for industrial process and gas transmission.