Abstract: CTOD tests were carried out for submerged arc weld seam of offshore platform structures based on BS7448 standard,and the Pop-in phenomenon was assessed and CTOD values were calculated.The results show that when Pop-in phenomenon appears,the fracture toughness CTOD(δ_Pop) will decrease several to more than ten times than that without considering Pop-in phenomenon.The research shows that Pop-in phenomenon is mainly due to local brittleness in crack tip,which can be caused by the slag inclusion in local zone of crack tip or the precipitation net in the grain boundary appeared in microstructure of local crack tip.In addition,when there were massive FSP(ferrite side plate) and B(bainite) in one seam of multi-pass weld,local brittleness was easily to occur and lead to Pop-in phenomenon if the crack tip just located in the zone of brittle microstructure and its propagation direction was parallel to the growth direction of brittle microstructure.