In this study, martensite transformation behavior of low transformation temperature (LTT) weld metals under unrestrained and restrained conditions was analyzed by thermal simulation technology. It is found that martensite transformation start (M-s) temperature and martensite transformation temperature range of LTT weld metal under restrained condition increased by 85 K and 50 K, respectively, compared with that of under unrestrained condition, which is attributed to the significant increase Gibbs free energy produced by restraint stress for martensite transformation. The increasement of M-s due to the effect of restraint stress is quantitatively described by Fe-X-C ternary regular solution-elastic energy model. Based on the transformation kinetics curve, the effect of restraint stress on martensite transformation of LTT weld metal can be divided into three stages, including stress-induced nucleation, transition and without effect stages. In initial stage, the martensite nucleates at a relatively high temperature with a low phase transformation rate due to the directional effect of restraint stress; in second stage with martensite transformation fraction range of 0.2-0.6, the elastic energy caused by restraint stress decreases significantly, and the chemical free energy of martensite transformation under restrained condition is lower than that under unrestrained condition, which results in the martensitic transformation rate of LTT weld metal under restraint condition being lower than that under unrestrained condition; in final stage, the small value of restraint stress has little effect on martensite transformation of LTT weld metal, and the transformation rate of martensite mainly depends on the content of austenite. This study provides a theoretical base for better understanding phase transformation and microstructure of LTT weld metal under restrained condition.