An acetylene detection system has been constructed with a tunable erbium-doped fiber laser (TEDFL) based photoacoustic spectrometer. Combining wavelength modulation and second harmonic signal detection technique, the system is able to effectively eliminate the background noise generated by absorption of the cell windows and the wall of the acoustic resonator. The system was applied to low concentration acetylene flowing measurement, through optimizing the systemic optics, acoustics and electron detection technique condition. The experimental results show that the second harmonic amplitude is directly proportional to gas concentration. Their linear pertinence coefficient is 0.999 53. The system sensitivity limit for acetylene detection is 0.3 ppm under atmosphere pressure with 100 ms time constant and 3.5 mW average optical power. The TEDFL was used as light source in stead of the DFB diode laser which is relatively costly. The system appears promising as a basis for developing cheap, practical, portable spectroscopy instruments for trace gas detection. The detection sensitivity could be greatly improved by using multi-path cell or increasing the laser power with EDFA.