Martensite-austenite (M-A) constituent formed during welding is generally recognized as an important factor to decrease the toughness of welded joint. In this article, the morphology and chemical composition of M-A constituent in the low carbon bainitic steel welded joint was analysed in detail by means of optical microscope, transmission electron microscope and scanning electron microscope with electron probe microanalysis. The experimental results show that the M-A constituent formed in the different sub-zones presents different morphologies and different amounts. The maximum amount of M-A constituent occurs in the coarse grained heat affected zone (HAZ). It is evident that the carbon atoms segregate on the M-A constituent and carbon concentration on the slender M-A constituent is higher than that on the massive M-A constituent. Meanwhile, the distribution profile of silicon on the M-A constituent shows an obvious inhomogeneity. Most of M-A constituents have a twinned structure and/or a high dislocation density. According to impact testing results, the crack initiation energy in the HAZ specimens deteriorates significantly because the large M-A constituent can assist the formation of cleavage crack. On the other hand, the coarse prior austenite grain in the HAZ lowers the crack propagation energy.