Without proper and well-managed encryption, digital communications interception is technically easy to tap over public networks, and it causes unauthorized disclosure of sensitive information and irreparable harm to individuals as well as organizations. To ensure that digital information remains complete, accessible and secure at all times, it needs to be reliably processed on security measures at both the sending and receiving ends. Signcryption like other aspects of a cryptographic system simultaneously provides the characteristics of both digital signature and encryption and can fulfill the basic security requirements of information confidentiality, integrity and availability in transmission. In this paper, we introduce a threshold signcryption scheme based on elliptic curve cryptography to combine the dynamic knapsack cryptosystem technique with the Chinese remainder theorem and achieve a more robust security cryptosystem for multi-document transmission. With the proposed composition scheme of threshold signcryption function, participants not only can employ the avalanche effect of unsystematic properties founded on the elliptic curve cryptography's point addition to chaotically generate preshared secure cipher keys against the counterfeiting or potential information leakage problems but also apply the advantages of a smaller key size and its high strength of security under the elliptic curve discrete logarithm problem assumption to carry out digital communications, which can be fast and reliably transmitted over a public channel.