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Digital signatures can guarantee the unforgeability and transferability of the message. Different from classical digital signatures, whose security depends on computational complexity, quantum digital signatures (QDS) can provide information-theoretic security. We propose a general method of random pairing QDS (RP-QDS), which can drastically improve QDS efficiency. In a way, our random pairing method provide a tightened result of security level of QDS. In the method, the parity value of each pair is used for the outcome bit value. We present general formulas for fraction of untagged bits and error rates of the outcome bits. Random pairing can be applied as a fundamental method to improve the QDS efficiency for all existing quantum key distribution (QKD) protocols. We take sending-or-not-sending (SNS) QDS and side-channel-free (SCF) QDS as examples to demonstrate the advantage of random pairing through numerical simulation. Similar advantage with random pairing is also founded with decoy-state MDIQKD and also decoy-state BB84 protocol. We study the RP-SNS-QDS with finite data size through novel optimization. The numerical simulation results show that the signature rate can be increased by more than 100% under noisy channel using our random pairing method.