A dynamic threshold key management scheme based on bilinear pairing without a trusted party is proposed in this paper. The system can dynamically add, modify and remove a member in MANET, and can change the threshold value according to the scale of the group. Adopting distributed framework generates the system key, which is constructed by all participants collectively, it is not like most presented schemes generated by Key Generation Center (KGC). In our threshold scheme, any participant can recover the system public key, but can not reconstruct the system secret key without any other t - 1 participants' help. The designated combiner (DC) can verify the correctness of shared secret during the recovery phase of system key. Simultaneously, the threshold polynomial based on elliptic curve cryptography (ECC) is obviously more secure and efficient than the previous schemes' polynomial based on prime-field. Additionally, we design a new feasible bilinear pairing based signcryption scheme, which is efficient in terms of both computational complexity and communication load and can realize security communication between any two participants. The security analysis shows that the proposed scheme is more feasible, secure and efficient compared with the previous schemes on Mobile Ad Hoc Network.