Abstract: Objective The aim was to address the shortcomings in lack of researches on K-TIG welding process and mechanism for 6 mm thick stainless steel plates. Methods K-TIG butt welding of 304 stainless steel plates with a thickness of 6 mm was carried out. Through a high dynamic range industrial camera, stability of keyholes and corresponding weld formation under different welding parameters was investigated. Results It was found that there were three common states of keyholes, welding leakage, stable perforation, and lack of penetration. The test results showed that when welding current was 450～525 A and welding speed was 300～400 mm/min, full penetration welded joints with stable welding keyhole and good single-side welding with double-side formation could be obtained. Conclusion On this basis, complete K-TIG butt welded joints of 6 mm thick S32101 duplex stainless steel were obtained by fine-tuning welding parameters. Microstructure of welded joints was characterized. It was found that different regions of welded joints were composed of ferrite and austenite, and the whole was still a dual-phase microstructure, but austenite content in weld was reduced compared with base metal. Mechanical properties of welded joints were tested, and it was found that the overall hardness distribution of welded joints was relatively uniform, and the average tensile strength of welded joints was 658.14 MPa.