Design evaluation is necessary and important for conceptual design because it is often a must for designers to select a design concept from a number of candidates for further development. Assembly devices are usually product specific and there need extensive efforts in developing a feasible conceptual design solution satisfying a number of design constraints. To get a suitable evaluation method for assembly devices in the conceptual design phase, it’s necessary to apply Fuzzy mathematics in the evaluation method and the corresponding algorithm. This paper proposes a fuzzy index evaluation method for the conceptual design of assembly devices, where a number of evaluation indexes specific for assembly devices are proposed and organized into various levels for the evaluation algorithm. Finally, an evaluation software platform is presented, together with a simple design case study to illustrate the application of the proposed evaluation methodology.