The small-angle neutron scattering(SANS)instrument, one of the first three instruments of the China Spallation Neutron Source(CSNS), is designed to probe the microscopic and mesoscopic structures of materials in the scale range 1–100 nm. A large-area ~3He tube array detector has been constructed and operates at the CSNS SANS instrument since August 2018. It consists of 120 linear position-sensitive detector tubes, each 1 m in length and 8 mm in diameter, and filled with ~3He gas at 20 bar to obtain a high detection efficiency. The ~3He tubes were divided into ten modules, providing an overall area of 1000 mm × 1020 mm with a high count rate capability.Because each tube is installed independently, the detector can be quickly repaired in situ by replacing damaged tubes.To reduce air scattering, the SANS detector must operate in a vacuum environment(0.1 mbar). An all-metal sealing technique was adopted to avoid high-voltage breakdown by ensuring a high-voltage connection and an electronic system working in an atmospheric environment. A position resolution of 7.8 ± 0.1 mm(full width at maximum) is measured along the length of the tubes, with a high detection efficiency of 81 ± 2% at 2 ?. Operating over the past four years, the detector appears to perform well and with a high stability, which supports the SANS instrument to finish approximately 200 user scientific programs.