Abstract: A new process of double-sided oscillation laser dislocation synchronous welding was proposed for autogenous laser welding of thick plates with big root face height. The backing welding experiments with different laser spacings were carried out, the finite element model of double-sided laser backing welding was established. The morphology of the molten pool was obtained and verified by comparison with the morphology of the test weld. On this basis, the welding thermal process and microstructure evolution of the coarse-grained region of the joint were analyzed. The results show that, the double-sided oscillation laser dislocation synchronous welding could achieve good penetration and forming of big root face height backing weld. Compared with single-sided laser backing welding, t_8/5 and t_8/3 of double-sided laser welding in the coarse-grained area increased; when laser spacing was not less than 50 mm, the coarse-grained region was re-austenitized. The microstructure of single-sided laser welding in coarse-grained area was lath martensite, the microstructure of double-sided welding was lath martensite and lath bainite, and the content of lath bainite gradually increased with the shortening of laser spacing. The double-sided oscillation laser dislocation synchronous welding was beneficial to improve the joint performance and reduce the tendency of cold cracking.