In 1996, Mambo et al. introduced the concept of proxy signature. However, a proxy signature only provides the delegated authenticity and doesn't provide the confidentiality. Chan and Wei proposed a threshold proxy signcryption scheme (denoted as Chan-Wei scheme), which extended the concept of proxy signature. In this paper, the authors demonstrate Chan-Wei scheme does not satisfy strong unforgeablity, strong nonrepudiation and strong identifiability. Based on Chan-Wei scheme, a nonrepudiable threshold proxy signcryption scheme with known proxy agents is proposed. The proposed scheme overcomes the weaknesses of Chan-Wei scheme. Completeness proof and security analysis of the proposed scheme are presented. In addition, compared with Chan-Wei scheme, the proposed scheme exactly finds out which proxy agents present bogus secret shadow or tamper secret shadow.