Harn et al. proposed a series of Diffie-Hellman key exchange protocols which are integrated into Digital Signature Algorithm in 2004. Recently, Phan pointed out that Harn et al.'s protocols cannot provide forward secrecy and key freshness, which are two standard security attributes that key exchange protocols should have. Phan also gave his improvement. In this paper we present a better improvement, which is more secure than Phan's scheme.