Recently, an improved digital signature scheme was proposed by He. However, there is a weakness in He?s scheme, where the forgers can forge valid signatures. In this paper, we show that there is also a drawback that a public key may have more than one corresponding secret key, where the verifier cannot determine who the real signer is. As a result, we improve He?s scheme to overcoming the above flaws. Moreover, we propose a message-passing protocol. In the proposed protocol, the delivered message can be extended by the valid senders, and the appended signatures are used to ensure the integrity of the message and to verify each signer of each sub-message.