Microstructure performance in the welding zone of T91 heat-resistant steel under the condition of TIG welding wasresearched by means of metallography, X-ray diffraction and scanning electron microscope (SEM). Experimentalresults indicated that microstructure of T91 weld metal was austenite + a little amount of δ ferrite when using TGS-9cb filler wire. Substructure inside the austenite grain was crypto-crystal lath martensite, on which some Cr23C6blocky carbides were distributed. The maximum hardness (HRC44) in the welding zone is near the fusion zone. Thereexisted no obvious softening zone in the heat-affected zone (HAZ). For T91 steel tube of φ63 mmx5 mm, whenincreasing welding heat input (E) from 4.8 kJ/cm to 12.5 kJ/cm, fracture morphology in the fusion zone and theHAZ changed from dimple fracture into quasi-cleavage fracture (QC). Controlling the welding heat input of about9.8 kJ/cm is suitable in the welding of T91 heat-resistant steel.