Abstract: comprehensive study the influence of preheating temperature on weldability of X100 pipeline steel. The results showed that with the increase of preheating temperature, X100 pipeline steel critical fracture stress increases gradually, HAZ hardness reduced gradually; As preheating temperature increases, the lath bundle of BF is more and more narrow and the distribution is more and more closely; As preheating temperature increases, the HIC features reduced gradually. Though the implant test method and heat affected zone the highest hardness method, this article measure the critical fracture stress and hardness value of heat affected zone of X100 pipeline steel under different preheat temperatures which is 80, 120, and 160℃, analysis the microstructure and fracture morphology. This paper research the influence of preheating temperature on weldability of X100 pipeline steel. The results showed that with the increase of preheating temperature, the critical fracture stress of X100 pipeline steel increases gradually and the hardness of HAZ reduced. The lath bundle of BF become narrower and closer, and the HIC features in the cracking zone decreased when the preheating temperature raised.