The inter-critically reheated coarse-grained heat-affected zone (ICCGHAZ) of X70 pipeline steel with different second peak temperature and heat input was simulated in this study by means of Gleeble3500 simulator. The volume fraction, size, shape and distribution of martensite–austenite (M–A) constituent were analysed. The toughness of ICCGHAZ and corresponding fractographs were examined. The results showed that the distribution of M–A was strongly influenced by second peak temperature, and M–A constituent with necklace structure at lower second peak temperature led to worse toughness. The volume fraction and size of M–A were strongly affected by heat input, the volume fraction of M–A constituent increased with the increase of heat input; the volume fraction and size of M–A were key factors of toughness deterioration; the interfacial energies and the initiation of crack were related to the shape of M–A constituent.