It has been the focus of study on improving the security and sensitivity of transformer differential protection recently. Because of its higher sensitivity， differential protection based on fault component has been widely used in practice. But traditional criterion of differential protection is impossible to improve both security and sensitivity of protection simultaneously with setting fixed characteristic curve. A new criterion for adaptive protection that can adjust the parameters of protection characteristic according to operation modes of power grid is proposed. It can reliably discriminate fault inside or outside region and has higher sensitivity and security than that of traditional differential protection based on fault component. The feasibility and superiority of this method are proved by dynamic simulation test on an ultra-high voltage transformer. Finally， the ability of the new criterion to avoid mal-operation under the circumstances of current transformer with partial transient saturation is analyzed.