A highly sensitive sulfur dioxide (SO₂) photoacoustic gas sensor was developed for the sulfur hexafluoride (SF₆) decomposition detection in electric power systems by using a novel 266 nm low-cost high-power solid-state pulse laser and a high Q-factor differential photoacoustic cell. The ultraviolet (UV) pulse laser is based on a passive Q-switching technology with a high output power of 28 mW. The photoacoustic signal was normalized to the laser power to solve the fluctuation of the photoacoustic signal due to the power instability of the UV laser. A differential photoacoustic cell can obtain a high Q-factor and reduce the gas flow noise in SF₆ buffer gas. The parameters of the SO₂ sensor system were optimized in terms of laser power and operating pressure. A 1σ detection limit (SNR = 1) of 2.34 ppb was achieved with a 1 s integration time, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 7.62 × 10⁻¹⁰ cm⁻¹WHz^−1/2.