The uneven impact toughness has always been an important problem in the production of low carbon bainitic steel plates. The microstructure uniformity of 07MnNiMoDR low carbon bainitic steel sheet in the width and thickness directions was studied. The results show that the grain size near the surface in the thickness direction is smaller and the hardness is lower; the grain size at the central position is larger, and the hardness is higher due to the segregation of carbon. In addition to the structure and hardness, large fluctuations also exist along the width direction. The main segregation elements of the tempered steel sheet at the 1/2 position of the thickness are carbon, sulfur, manganese, and niobium, etc., and exist in the form of MnS and NbC compounds, which significantly increase the probability of intergranular fracture. Element segregation also affects the uniformity of the tempered structure.