This paper presents a Need-Function-Principle- Structure (NFPS) model of engineering design and its application to conceptual design of a dark fiber identifier. Firstly, the NFPS model was presented, and the core concepts of this model were introduced. Secondly, the process of conceptual design based on the NFPS model was described. Finally, the NFPS model was applied to conceptual design of a dark fiber identifier, which demonstrated the feasibility in utilizing the model to conceptual design of multidisciplinary systems. In addition, the NFPS model can be used in embodiment design and design knowledge reuse for further study.