Tunable diode laser absorption spectroscopy (TDLAS) technique is a method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser used to obtain the absorption spectroscopy in the characteristic absorption region. The dissolved gas analysis (DGA) is essential for the insulation state assessment and the fault diagnosis of transformers. In order to monitor the fault gases CO2 and C2H2, a set of system of simultaneous online monitoring the concentrations of CO2 and C2H2 is designed, and this system makes use of TDLAS technique combined with multiple reflection pool, harmonic detection and time-division multiplexing optical switch technique. The system has benefits of high accuracy, selectivity, fast response and the nature of explosion-proof, which is suitable for power transformer fault gases on-line monitoring.