Abstract: Design and quality control of flat steel ingots is a key for producing extra thick plates by smelting,casting and rolling processes. Flat steel ingots with ultra-large width to thickness ratio can be directly rolled without cogging,thus simplifying the production process and reducing costs. The air gap distribution,solidification process and shrinkage defect distribution of flat steel ingots with ultra-large width to thickness ratio were studied by using numerical simulation.The effects of different width to thickness ratio and preset arch parameters on the temperature field and quality of the ingots were analyzed.The results show that during the solidification of the ingot with a large width to thickness ratio,the difference of air gap thickness between the middle of the broad surface of the ingot and the 1/4 position of broad surface is more than 1mm,and then two hot spots are generated inside the ingot.With the increase of the width to thickness ratio,the phenomenon of two hot spots is more obvious,and the shrinkage porosity defects increase.In addition,with the increase of the pre-arch height,the phenomenon of the two hot spots is less obvious,and the volume of shrinkage porosity decreases.When the arch height is 80 mm,the quality of the ingot with ultra-large wide to thickness ratio is good.