A (t,n) threshold proxy signature scheme enables a proxy signer to sign messages on behalf of the original signer. Hwang et al. recently proposed an practical (t, n) threshold proxy signature scheme. In this paper we propose an efficient threshold proxy signature scheme based on RSA signature scheme, which improves security of their scheme. Our scheme divides the document M into m blocks and add some redundant bits to each message block. It is embedded in the proxy signature issuing phase to avoid the conspiracy attack. The proposed signature scheme satisfies all the secure requirements and is efficient.