The essentiality of maintenance assessment in the design stage is analyzed, and the existent problems of conventional maintenance assessment method are illuminated. Maintenance Assessment Method based Digital Mock-Up (MAMDMU) is founded, and the process of MAMDMU is established. The key techniques of MAMDMU are analyzed, and it includes virtual maintenance modeling, maintenance process simulation based physical attribute, and maintenance assessment based simulation result. The makeup of immersion virtual maintenance environment is analysed. For maintenance assessment characteristic, system architecture of maintenance assessment system is illuminated based MAMDMU, and it includes design knowledge database, design model database, virtual maintenance database, product design, maintenance modeling, maintenance simulation, maintenance assessment, visualization, and so on. Every module function of maintenance assessment system is analyzed, and the realization method of every module is depicted. An application example of MAMDMU is illuminated based maintenance assessment method and system, and the future imperative studies for MAMDMU are discussed based the present work.