Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) are used to study the interface microstructure and formation mechanism of laser welding joints of TRIP590 steel. Analyzes of SEM indicate that the microstructure of weld zone is matensite, and the microstructure of heat affected zone (HAZ) are bainite and ferrite. The closer of the weld, the more martensite HAZ has. The results of EBSD analysis show that there is no obvious preferred orientation and uniform sizes of grains in base zone, and most of grain boundaries are high angle grain boundaries. The grain size of HAZ is unequal and the bainite has the same or similar orientation. The laths size of weld zone is coarser and it has obvious texture. Retained austenite is dispersed in grains and grain boundaries. The orientation difference of grain boundary of weld and HAZ is low-angle boundary which is between 1°~5°. A large amount of low-angle boundary would lead to the plasticity loss of weld and HAZ compared with base metal.