Certificateless signature scheme is a practical solution to confront the drawback, Key Generation Center (KGC) being able to forge the signature of a user, of an identity based (ID-based) signature scheme. Lots of previous research results have shown the security models and the generic constructions for certificateless signatures. However, most of them did not satisfy Girault’s level-3 security which the conventional public key infrastructure (PKI) can achieve. Until 2007, Hu et al. introduced a generic construction and security model that can fulfill the requirement of Girault’s level-3 security. Recently, Du and Wen proposed a certificateless shortsignature scheme which is more computation efficient than the previous ones. But a flaw in security proofs and lack of Girault’s level-3 security can be still found in their scheme. In this paper, a cryptanalysis on Du-Wen scheme and an improved scheme will be presented, and we also provide formal proofs to demonstrate the security of the proposed scheme.