Abstract: Welded joint's tensile strength of high Ti and Nb precipitation strengthing high strength steel with MAG(80%Ar+20%CO₂) was tested.The results show that the welded joint strength decreases with the increasing of the heat input.The hardness of the HAZ is lower than the base metal, and the HAZ has softening behavior.The growth of the grain and dissolution of the(Ti, Nb, Mo)(C, N) second phase particle(< 10 nm) in CGHAZ weakend the precipitation strengthing.The particle without dissolution hold C and Mo, which weaken the stability of the overcooling austenite.The CGHAZ can not have lath martensite or bainite with high hardness, but has granular bainite with low hardness.The decreasing of the precipitation strengthing can not be balanced by structure strengthening, which leads to softening behavior in CGHAZ.The coarsing of the(Ti, Nb, Mo)(C, N) second phase particle(< 10 nm) in FGHAZ, which offsets the critical size with best precipitation strengthing, weakened the precipitation strengthing, led to softening behavior in FGHAZ.