SO₂ and NO₂ are the most important pollution in atmosphere. An optimized long path (LP) differential optical absorption spectroscopy (DOAS) system of high light intensity at an ultraviolet (UV) wavelength is proposed and used to measure the concentration of SO₂ and NO₂ simultaneously. In contrast to the traditional DOAS, the system adopted a Y-type optical fiber structure instead of a combination of mirrors in the telescope. The UV light intensity test shows that the light intensity of UV can arrive to above 80% of the max measuring range when the light path reaches 135 m, and the integral time of the spectrograph is only 15 ms. The system is proved to be efficacious through laboratory calibration. The maximum error of SO₂ calibration is 4.19%, and is 5.22% for NO₂. The error of the SO₂ and NO₂ mixture calibration is within 10%. Field measurement is implemented in a wastewater treatment plant in winter. The measurement light path is 738 m. The concentration of SO₂ varies from 6 μg/m³ (2.26 ppb) to 20 μg/m³ (7.52 ppb), and the concentration of NO₂ varies from 100 μg/m³ (53.2 ppb) to 200 μg/m³ (106.4 ppb) approximately. The results are in accordance with the data from a monitoring station nearby in magnitude order and variation tendency mostly.