The presence of acetylene in transformer oil is often an important feature of discharge or high temperature faults. The concentration of acetylene gas can show whether partial discharge or arc occurs in voltage transformer. Therefore, the detection of trace acetylene gas content is of great significance for on-line evaluation of transformer operating status and prediction of transformer life. This paper adopts tunable diode laser absorption spectroscopy (TDLAS) technology to measure acetylene in transformer oil with high sensitivity. Compared with the traditional TDLAS system, this work selects an anti-resonant hollow-core optical fiber as gas chamber, which has a hollow core for gas filling and a low loss for light-wave transmission by anti-resonant principle. This fiber has advantages of avoiding energy loss caused by light coupling and scattering, leading to light-matter interaction enhancement, as well as gas consumption reducing. Experiment illustrates that the lower limit of detection for acetylene reaches 25 ppm.