Abstract: Based on the actual project and practical application, an analysis is made of the definitions, methods and their application in the value assessment of engineering. Then an optimal value theory is put forward and innovatively applied to the equipment value assessment. In addition, we put forward the definition of assessment coefficient of equipment value and analyze the equipment value model, assessment model, optimal value computing method and the most appropriate functional level deciding method, which provides important theories and methods for solution argument, value assessment, R & D order and scientific decision making.