The analysis of influence to crop length and width deviation by different broadside rolling ratio and longitudinal rolling ratio in broadening phase and finish rolling phase in plate mill was developed. As to different broadside rolling ratio and longitudinal rolling ratio, an optimization method of edging amount was put into forward, in which optimal edging amount was confirmed so as to obtain minimum trimming loss by the stagnation point . From actual application it shows that this method can decrease product trimming loss by more than 15%, and it can increase plate product yields greatly.