To improve the correctness and feasibility of the implementation of multilevel security in the distributed environment, a distributed multilevel security core architecture — Distributed Trusted Computing Base (DTCB) was proposed. DTCB was divided into three layers, TCB of System layer, TCB of Module layer and TCB of Partition layer, finer multilevel control granularity was realized step by step, greatly reducing the complexity of the implementation of multilevel security in the distributed environment. At last, based on the composable noninterference model, the security of DTCB was formally proved. The result shows that DTCB assures the multilevel security of distributed system as a whole.