Multi-channel isolation transmission based on fragmentation of information data is one of the common methods to ensure the security of information transmission, but it lacks the theory of provable security and evaluation methods. A multi-channel fragmentation transmission system is formally defined firstly, including data encryption, fragmentation multi-channel transmission and recombination functions. Then from the data leakage probability analysis in the process of multi-channel transmission, the provable security level of multi-channel fragmentation transmission technology is defined. A set of security level evaluation methods for multi-channel data fragmentation transmission is established to verify the effectiveness of the security level evaluation method from the theoretical point of view. This paper provides theoretical guidance for the realization of multi-channel data fragmentation transmission technology and guidelines for the security evaluation of multi-channel data fragmentation transmission system. The proposed method can also be applied to the security analysis of data fragmentation storage and transmission.