A portable laser-flash photolysis Faraday rotation spectrometer for measuring atmospheric total hydroxyl (OH) reactivity was developed, with optical box dimensions of 130 cm × 40 cm × 35 cm. It features a pump–probe multi-pass cell with a high overlapping factor of 75.4 %. The instrument’s precision and uncertainty are 1.0 s^-1 (1σ, 300 s) and within 2 s^-1, respectively. This portable, cost-effective instrument expands current measurement capabilities and is convenient for field applications.