Under-frequency load shedding（UFLS）scheme with starting frequency and time delay is widely used in China. UFLS scheme designed with traditional method is rough and difficult to adjust because of model simplifications. To satisfy the requirement of maximum system power deficit，a new method is proposed to optimize UFLS schemes based on transient frequency deviation security（TFDS）. Criteria of basic levels are treated as two-element tables of transient frequency security. Transient and steady frequency constraints are quantitatively considered with TFDS. Considering the effect of voltage dynamics on UFLS，voltage variation is converted to frequency deviation to improve adaptability of the UFLS scheme. Simulations show that the proposed UFLS scheme can be properly designed to effectively control frequency deviation. Frequency stability information can also be obtained with this method.