In order to investigate the relationship between temperature distribution and shape control for thin medium plate, based on the production of a 3800 mm finishing mill, a high-precision temperature field prediction model of the rolled piece was established using the control volume heat balance method. The model is used to calculate the temperature field in the thickness, width, and length directions of the piece, and the results are used to analyze its effect on the shape control. By comparing the calculated results with the measured data, it can be seen that the deviation of the calculated results of this model was within 1%. Firstly, considering the large temperature difference in the thickness direction of the piece, an optimization coefficient of each pass for rolling force calculation was proposed to improve the accuracy. Secondly, the influence of the temperature difference in the width direction on the wedge shape of the piece is quantitatively analyzed, and the control standard of the temperature difference within 20 °C was proposed. Thirdly, the influence of the temperature difference in the length direction on the shape control of the piece is quantitatively analyzed, and the tail edge wave problem was solved by increasing the rolling temperature and limiting the length of the piece. The above research has been applied in production, and the shape quality of different specifications of plate has been improved.