Digital signature is a key technique in information security, especially for identity authentications. Compared to classical correspondence, quantum digital signatures (QDSs) provide a considerably higher level of security. At present, its performance is limited by key generation protocols, which are fundamentally limited in terms of channel capacity. Based on the idea of twin-field quantum key distribution, this Letter presents a twin-field QDS protocol and details a corresponding security analysis. In its distribution stage, a specific key generation protocol, the sending-or-not-sending twin-field protocol, has been adopted. Besides, we present a systematic model to evaluate the performance of a QDS protocol and compare the performance of our protocol to other typical QDS protocols. Numerical simulation results show that the new protocol exhibits outstanding security and practicality compared to other existing protocols. Therefore, our protocol paves the way toward real-world applications of QDSs.