The structure of DC transmission system is complexity, and the characteristics differ from components to components. It is difficult to adopt a single reliability assessment method for HVDC transmission protection system to be comprehensive, rapid and accurate. A reliability assessment method based on hierarchical modeling is proposed. At first the structure of DC transmission protection system is analyzed. It is divided into system layer, module layer and component layer. Then according to the characteristics of each layer, appropriate assessment methods are selected to assess each layer of the protection system. And corresponding reliability index is set the to realize the connection between each layer. Taking a DC transmission project as an example, the comprehensive reliability assessment of the protection system is carried out. Thereby the correctness and effectiveness of the proposed method is validated.