On the basis of the wavelength scanning and spectrum modulation techniques of an ultra-narrow-linewidth semiconductor laser, a kind of multi-component gas online detection system with high precision was designed. A semiconductor laser with ultra-narrow-bandwidth and tunable wavelength was utilized as a light source and a novel series gas cell was also used to compress the horizontal space and increase the optical path of the system. By taking the sawtooth signal to modulate the spectrum of laser and using space division multiplexing (SDM) and time division multiplexing (TDM) techniques, the detection of multi-component gases(CO, CO2, CH4) based on ultra-narrow-linewidth laser characteristics was achieved. The experiments indicate that the relative measurement error is less than 2% and linearity relevance coefficient is 0.99 for the three kinds of gases. Furthermore, the system dynamic response time is less than 10 s by filling a volume of multi-component gas into the gas cell gradually. Finally, the result of long time stability monitoring shows that the maximum concentration fluctuation is less than 0.02%. The system has advantages of high accuracy, good stability and quick response, and is suitable for the fault gas monitoring of power transformers in real time.