Pulsed laser resonant photoacoustic spectroscopy was applied for detection of highly toxic SO₂ with 266 nm as the excitation source. An extra-cavity longitudinal resonant cell, was designed and fabricated to enhance the sensitivity of the system, which is capable of detecting the trace amount of SO₂. As a process of signal-to-noise ratio optimization, the parametric dependence of the PA signal was carried out and the sensitivity achieved with our system was as low as 4 ppb, which is considered to be a sufficient level for the detection of ambient SO₂ in the atmosphere. This study could be an antecedent for the design of a portable SO₂ sensor.