Abstract: Key-hole tungsten inert gas（K-TIG） welding was used for butt welding test of 09 MnNiDR low-temperature steel plate with the thickness of 10 mm, the cross section of the single pass welds at different parameters were studied. The welded joints before and after post-weld heat treatment were studied from surface characterizstics, macroscopic morphology,microstructure and mechanical properties. The results show that with the increase of the welding current, the weld penetration improves; with the increase of the assembly clearance, the weld width increases. The microstructure of the base metal is ferrite and pearlite. The bainite precipitates from the microstructure of K-TIG weld zone and cap weld zone; the microstructure of the cap weld zone is mainly acicular ferrite. After post-weld heat treatment, the columnar crystal in K-TIG weld zone turns to equiaxed grains, part of the acicular ferrite in cap weld zone transforms to granular ferrite. Under the optimum process parameters, the tensile strength of the welded joint before and after post weld heat treatment are 524.1 MPa and 481.4 MPa,respectively. The impact toughness of the K-TIG weld zone is poor, which is only 6.7 J at -70 ℃, and it improves obviously to 94.8 J after post-weld heat treatment.