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Wen et al. recently showed Hwang's threshold proxy signature scheme based on RSA has some mistakes of equation and which makes it unsecured. So they modified Hwang's scheme. However, we find the core of the problem of the value of Lagrange coefficient is still not solved and the right of proxy signature combiner isn't defined clearly in the modified scheme. We modify Hwang's scheme once more and our modified scheme needn't compute the inverse when we compute the value of the Lagrange coefficient. Furthermore, in our modified scheme proxy signature combiner don't know the Euler function value Phi(N0) which can prevent combiner and partial signer from the conspiracy attack. The improvement makes the scheme perfect and practical.