This paper mainly discusses the definition and extraction method of lip crown. According to the motion law of the upper and lower lips, the convexity of upper lip and lower lip are respectively defined as the Euclid distance of the upper lip or lower lip along the upper or lower incisors. The motion capture device is used to acquire the three-dimensional coordinate information of the movement of facial markers in the process of pronunciation. The singular value decomposition method is used to eliminate the head rigid body movement and the mandibular opening movement. The reference points placed in facial bones are used to calculate the space of upper and lower incisors respectively Position, using the upper lip and lower lip coordinates of the outer edge of the upper lip or lower lip crown. The results show that this method not only performs well on three-dimensional lip data, but also applies to two-dimensional lip data.