The complicated interactions among tasks lead to iterations in product design and development. To maintain the market share, enterprises should effectively manage their product development project and bring their products to market as early as possible. The objective of the paper is to investigate how to analyze design iteration. Firstly, the improved parallel iteration model based on work transformation matrix (WTM) is proposed, which includes the modification of both the time of iterations at each iteration stage and the number of iterations as well as considering the effects of incompleteness of information transfer on the time of iterations. Secondly, the GA is adopted to find the optimum scheme of time and efforts of coupled task set. Finally, an example deriving from camera design and development process is used and the results show that the method presented in the work is not only efficient but also effective.