Abstract: The oblique 45° welding without wire filling of 3.15 mm thick CT80 steel plate-to-plate was carried out by using automatic TIG welding, and heat treatment was carried out for the automatic TIG weld area.The heat treatment process was heating at 920 ℃for 60 s, then air cooling was adopted, or cooling from 920 ℃to 450-400 ℃then forced air cooling or water cooling was adopted.The effect of the heat treatment on the microstructure and properties was studied.The results show that, after heat treatment the yield strength of the welded joint decreases obviously, and the tensile strength and elongation slightly decrease.Different cooling methods have no obvious regular influence on the tensile performance of CT80 steel strip plate-plate welding joint. The microstructure of the weld and heat affected zone transforms from coarse granular bainite to acicular ferrite and proeutectoid ferrite after heat treatment, the hardness decreases and the weld hardening phenomenon is weakened. After heat treatment, the tensile fracture of the sample occurs at the base metal 10-15 mm away from the weld center, because the base material at the fracture undergoes recrystallization annealing during the heat treatment process, and the microstructure changes from polygonal ferrite + granular bainite to acicular ferrite+proeutectoid ferrite with large grain size, which reduces the hardness and strength.