The problem of fair exchange is one of the fundamental problems in secure electronic transactions and digital rights management. Recently, Park etc [Constructing fair exchange protocols for E-commerce via distributed computation of RSA signatures] present an optimistic fair-exchange protocols based on RSA, Dodis and Reyain [Breaking and Repairing Optimistic Fair Exchange from PODC 2003] analyze the vulnerability of Park's scheme and present an optimistic fair-exchange protocols based on GDH. This paper point out that Dodis and Reyain's scheme is also insecure and inefficient. This paper presents a multi-signature scheme based on DSA, describes a novel method of constructing very efficient fair-exchange protocols based on improved DSA signatures. Comparing with [Constructing fair exchange protocols for E-commerce via distributed computation of RSA signatures, Breaking and Repairing Optimistic Fair Exchange from PODC 2003], the protocols of this paper are more secure and efficient.