Transformer insulating oil uses paraffin (C_nH_2n+2) as the main chemical component. Due to electric arc, discharge, overheating, moisture, etc., the chemical bonds are broken, and characteristic gases (methane, ethane, ethylene, acetylene, carbon dioxide, carbon monoxide) are generated during the long-term operation of the transformer. Therefore, Multi-component gas will be dissolved in transformer insulating oil, so an online detection device for multi-component gas is needed to ensure the normal operation of the transformer. According to the assembly requirements of the power industry, this paper conducts an online detection device of 6 types of fault characteristic gases based on near-infrared tunable diode laser absorption spectroscopy (TDLAS). Based on the near-infrared absorption bands of 6 types of fault characteristic gases, the device selects four near-infrared lasers at 1 580, 1 654, 1 626 and 1 530 nm respectively. It uses the time-division multiplexing technology of time-sharing scanning to achieve fast time-sharing of multi-component gases Sequentially detect and adopt wavelength modulation technology to eliminate the cross-interference of the background gas. The designed near-infrared TDALS multi-component gas detection device mainly detects methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), acetylene (C₂H₂), carbon monoxide (CO) and carbon dioxide (CO₂). The developed experimental device is verified with the traditional transformer oil dissolved gas method (transformer oil meteorological chromatographic measurement method), and the working condition stability test is carried out. The measurement range of acetylene concentration is 0.5~1 000 μL·L^-1. When the range is less than 5 μL·L^-1, the maximum measurement error is less than 0.8 μL·L^-1. When the range is 5~1 000 μL·L^-1, the maximum error is below 6 μL·L^-1; the concentration measurement range of methane, ethane, and ethylene is 0.5~1 000 μL·L^-1, the maximum measurement error is less than 6 ppm; The measurement ranges of CO and O₂ are 25~5 000 and 25~15 000 μL·L^-1, and their maximum measurement errors are below 2 and 20 μL·L^-1 respectively. The designed near-infrared TDALS multi-component gas detection device can be used for online detection of dissolved gases in transformer oil, and the measurement meets the requirements of online detection. It can operate stably and adapt to harsh working conditions. The successful design of this on-line detection device provides practical experience for on-line measurement of dissolved gases in the detection of transformer oil.