A mobile ad hoc network has the computational and energy constraints, the general threshold cryptography does not suit it. In this paper, we consider a new kind of proper public encryption to the ad hoc networks: threshold broadcast encryption. A new construction of identity-based threshold broadcast encryption is proposed. In the proposed scheme, any user can dynamically join the system as a possible recipient, and the sender can dynamically choose the set of recipients and the threshold value t. Our new scheme achieves constant size private keys and O(n-t)-size ciphertexts. This is the first scheme which achieves short ciphertexts and full security in the standard model. Finally, we also show that it is provable security under n+1-Weak Decision Bilinear Diffie-Hellman Exponent (n+1-wDBDHE) assumption.