A brief description of various low-temperature steel used in liquefied natural gas (LNG) pipeline projects and its welding processes is given. For domestic Ni9 steel under different welding heat inputs, the influence of the secondary welding thermal cycle on Ni9 steel heat affected zone is discussed, as well as the structure characteristics and embrittlement tendency during the welding process. The research results show that single-pass welding is not suitable for Ni9 steel. However, in multi-pass welding process, the tempering effect of the subsequent welding can improve the low-temperature toughness well; recrystallization of the coarse grain structures is a key factor to improve the low-temperature toughness of Ni9 steel.