Abstract: In order to study the effect of welding peak temperature on softening of single-pass welded joint heat affected zone of X80 pipeline steel with excellent deformability, a thermal simulation testing machine was used to study the effects of peak temperature on microstructure and mechanical properties at 500-1 300 ℃ by simulating single-pass welding test. The results show that, when the temperature was 500-700 ℃, yield strength and impact toughness of the welded joint increased, tensile strength and hardness decreased, the content of pearlite decreased, black M/A island appeared and its distribution was along the grain boundaries. When the temperature was 800-1 000 ℃, softening phenomenon appeared, yield strength, tensile strength and impact toughness all reduced, the hardness drop was significant, the contents of M/A island gradually increased; The reasons for softening was incomplete recrystallization, the microstructure of matrix was given priority to with ferrite, and with the increase of temperature, carbide dispersion degree decreased, strength and hardness decreased. When the temperature was 1 100-1 300 ℃, yield strength and tensile strength of welded joint increased obviously, percentage reduction of area significantly reduced, hardness increased, the microstructure consisted of lath bainite and ferrite, M/A island transformed from fine and disordered arrangement to long striped and ordered arrangement.