This paper mainly focuses on improving the product performance by taking user factors into consideration during the design phase. Firstly, the impact of design modification on the product lifecycle and the importance of the user in the engineering design process is discussed. Secondly, some reasons of why simply carrying out a socio-technical analysis hardly work on changing the design result are explained, what kinds of the data should be concerned by designer and how to collect data are discussed. Last, some helpful methods are introduced. Considering the costs of modifications are usually very expensive and it may make the product (system) more complex, a novel idea is proposed, which to generate a user manual by analyzing the interaction between product behaviour and user behaviour to help designer to minimize the possibility of design modification after prototyping phase.