Abstract: A T-type multi-pass photoacoustic sensor was designed for the needs of fault diagnosis within gas-insulated equipment. The sensor, combined with a near-infrared laser with a center wavelength of 2333.72 nm, achieves trace detection of the characteristic fault gas CO in SF₆ gas-insulated equipment. The optical path length was effectively extended by optimizing the structure of the T-type photoacoustic cell and combining it with the multi-pass cell. The experimental results show that the multiple reflection design improves the photoacoustic signal intensity by 14 times compared with the single reflection, which significantly improves the detection sensitivity. The effect of gas pressure on the frequency response of the system was analyzed and tested by simulation using the finite element method. With the increase of gas pressure, the resonance frequency of the photoacoustic cell decreases, and the amplitude of the photoacoustic signal increases. The photoacoustic signal amplitude at 0.4 MPa gas pressure shows a 1.4-fold enhancement compared to that at 0.1 MPa gas pressure. The response of the detection system for CO per unit volume fraction under SF₆ background gas is 0.2 μV, and the minimum detection limit reaches 1.50×10⁻⁷.