Abstract: The concept of proxy signature, introduced by Mambo, Usuda, and Okamoto in 1996, allows a designated person, called a proxy signer, to sign on behalf of an original signer. Mambo et al. show that proxy signature should have properties of proxy signer’s deviation, unforgeability, verifiability, distinguishability, identifiablity, and undeniability and give three types of delegation: full delegation, partial delegation and delegation with warrant. Sun, Lee and Hwang proposed a threshold proxy signature scheme （denoted as S-L-H scheme）, which overcame disadvantages of the schemes proposed by Zhang and Kim et al , respectively, and gave an improved scheme. It is shown here that S-L-H scheme is insecure against the public key substitution attack and an improved threshold proxy signature scheme is proposed. Furthermore, using zero knowledge idea, a general approach to withstanding the public key substitution attack is given. The improved scheme has advantages of nonrepudiation, not using secure channel, being able to resist the public key substitution attack and collusion attack etc.