Digital design is an important tactic for mass customization. It is a key technique to establish an effective design model. Through analyzing design knowledge, a method of digital design based on a three-level-drive model which included demand parameters, key parameters and structure parameters was proposed. Demand parameters were the input of first level drive, where product key parameters were obtained through product scheme design system and optimization model. Key parameters were the input of second-level drive, where a hierarchical strategy was advised to establish constraint network model, so that detail structure parameters of parts can be derived. Detail structure parameters were the input of third level drive which could drive parametric geometry model and remote finite element analysis model to validate the design. Web-based technology was also employed to facilitate the distributed digital design. The whole strategy and method were put into practice in a crane factory, and proved to be applicable and effective.