In this paper, the effect of transverse thickness difference and width-to-thickness ratio on the critical unstable shape of cold-rolled strip is studied computationally and experimentally. Firstly, the influence model under actual working conditions is developed by referring to the basic relationship between transverse thickness difference and shape under ideal conditions. After that, the combined influence of transverse thickness difference and width-to-thickness ratio on the critical unstable shape is analyzed based on the elastic sheet stability model. Experimental validations demonstrate that during the production of ultrathin strip steel with different width-to-thickness ratios, the loading roller shapes should be fine adjusted according to the transverse thickness difference of input strips. Therefore, the variation of transverse thickness difference of output strips can meet the requirement of shape stability, so as to obtain fine shape.