Abstract: The lightening structural design of mechanical products aims at designing qualified mechanical products using the least design resources to reduce product costs, and to improve the performance-price ratio and capability for market competition. The essence of lightening design is the rational distribution of material mass in a geometrical space among components. The Guide-Weight method (GWM) is effective for structural optimization and can be used in lightening structural design of mechanical products. GWM is applicable to the optimization of component size for various structures, including diverse kinds of finite elements and optimization of geometrical shapes, as well as topology optimization. The objective functions and constraint functions of GWM can involve various kinds of structural static and dynamic behaviors. In this paper, firstly, the GWM's mathematical model and solution with abroad universality that can be used in lightening design of various mechanical products are presented. Then a square root envelope function is discussed, which converts the enormous stress or displacement constraints into single strength or precision constraint. A practical GWM model and solution with a single behavior constraint is also presented, which is suitable for the lightening design of most mechanical products. Finally, the applied lightening-design examples of two mechanical products are demonstrated, and the effectiveness and broad applicability of GWM are verified.