As every node could work on different frequencies in cognitive radio networks, cognitive nodes must consult a shared channel with other ones to communicate before sending the data. The method of negotiating the shared channel in Ad hoc cognitive radio network is that cognitive nodes give the plaintext on using frequencies, in other words, neighbor nodes knows the all information about this node' frequencies. However, compromise nodes make good use of the information of using frequencies to attack cognitive nods. To solve the problem, this paper the method based on secure multi-party computation, firstly the question is modeled with Millionaires' Problem, secondly the protocol of shared channel securely is designed. The method not only compute the accurate shared channel without revealing the information of using frequencies and the support of trust third party, but also protect the attack with the shared spectrum information.