Abstract: X120 pipeline steel has excellent comprehensive mechanical properties. The continuous cooling transformation (CCT)microstructure of X120 pipeline steel was studied using optical microscope and transmission electron microscope. The CCT curve under conventional heat treatment and SHCCT curve of coarse grain heat affected zone under laser-arc hybrid welding were established by thermal simulation equipment. The experimental results show that with the increase of cooling rate, the microstructures of the thermal simulated coarse grain region are granular bainite, bainite ferrite and lath martensite in turn under the condition of laser-arc hybrid welding. The rapid heating during the laser-arc hybrid welding increases the phase change lines of A_c1 and A_c3. The short residence time at high temperature inhibits austenite grain growth, therefore, the formed microstructure is refined. The M_s transformation temperature under the condition of laser-arc hybrid welding increases and the transformation temperature range of martensite is expanded.Thus, the formed lath martensite undergoes self tempering and the quenching stress reduces, which can improve the strength and toughness, and enhence the cracking resistance.