Based on a wavelength scanning technique and the characteristics of ultra-narrow-linewidth laser, a kind of cavity enhanced absorption gas detection system with high sensitivity is designed. A semiconductor laser with ultra-narrow-linewidth and tunable wavelength is used as a light source, and an optical cavity which consisted of two mirrors with high reflectivity is also used as an absorption cell in this system. The laser frequency overlaps with one of cavity modes via scanning the cavity length and the input laser is switched off by using the laser detuning technique.Then, the ring-down detection of acetylene with low concentration is achieved. With the cavity enhanced absorption technology, spectra of acetylene near the region of 6 518.824 cm-1 and laser ring-down time at different concentrations are measured and also analyzed. The experiments for concentration measurement indicate that the relative error is less than 2.5%, relevance coefficient is 0.999 and the lowest detection limit is 2×10-6. By filling a volume of acetylene into the gas cell gradually, the system dynamic response time is less than 10 s. Obtained results show that the system has advantages of good accuracy, high sensitivity and quick response, which can be used in the fault gas on-line monitoring for power transformers in real time.