Abstract: The finite element simulation of the backing welding of the high strength steel plates was built by using MSC. Marc and the effects of the reserved groove gap on backing weld root fusion was investigated based on the welding temperature field. And the results of simulation were verified and supplemented by the experiments. The results showed that, under the low alloy high strength steel required welding heat input, when groove gap size was less than 2 mm, the backing weld root appears incomplete fusion defects. When the gap increases to 2-4 mm, the backing weld has single face welding-double face forming. When the gap increased to more than 5 mm, the heat resource could not reach to the base metal of the groove sidewall and the weld root appeard incomplete fusion, and was necessary to use swing welding with high heat input for good fusion.