In order to solve the problem of low accuracy and sensitivity in multi-component trace gas detection,a kind of multi-component trace gas detection system based on wavelength scanning and photoacoustic spectrometry is designed.A semiconductor laser with ultra-narrow-linewidth and tunable wavelength is utilized as light source and a first-longitudinal resonant photoacoustic cell with multi-optical path is also used in this system.By taking the sine signal to modulate the wavelength of laser and combining the second harmonic signal detection technique with characteristics of ultra-narrow-linewidth laser,the detection of low concentration multi-component gases is achieved.The system is able to effectively eliminate the background noise and the influence of intensity fluctuation.The experiment results indicate that the relative measurement error is less than 2% and the linearity relevance coefficient is 0.99.The results of long time stability monitoring also show that the maximum relative fluctuation is less than 1.5%.Finally the sensitivity detection limits are determined to be:5×10-6 for acetylene,12×10-6for carbon monoxide and 15×10-6 for carbon dioxide.The system has advantages of high accuracy,high sensitivity and good stability,which can be used in power transformer fault gases on-line monitoring in real time.