Abstract: Optimization method has been well researched and developed in the past few decades. Studies have been also carried out on multi-disciplinary, multi-point, multi-objective, multi-constraint, and robust optimization as well as other optimization approaches. However, practical applications of these optimization methods still have difficulties in the engineering design. Therefore, by reviewing the application history of optimization methods in industrial design and analyzing the difficulties and problems in the application, "man-in-loop" is introduced as a methodology for applying optimization methods in practical engineering design. The concept, realization, and evolution of "man-in-loop" are explained with examples of aircraft aerodynamic design. Due to the significant development of artificial intelligence (AI) in non-linear regression and data mining, it is quite possible that the human role in "man-in-loop" can be replaced by the AI. According to some examples given by the present approach, potential directions for the development of engineering optimization design are discussed.