Ethylene (C2H4) is a common chemical material in the petroleum and chemical industry. It is explosive and often used as identifier for coal spontaneous combustion. An inexpensive photoacoustic spectrometry based on near infrared tunable diode laser is developed to detect the ethylene with low concentration. By analyzing the near-infrared absorption line of ethylene, we choose absorption line of ethylene at 1620.44 nm as measuring object. By combining wavelength modulation absorption technique, resonant frequency of photoacoustic cell is used as modulation frequency, and the second harmonic signal of absorption line is used to realize inversion of ethylene gas concentration. The results show that the measurement accuracy of ethylene by the system is 0.688%. The detection limit of the system is up to be about 1.16×10-5. A continuous measurement for a gas sample in half an hour shows the stability of the system. All the results represent the application value of the system for trace detection of ethylene.