The reverse modeling solution and its key technologies about complex part with internal cavity are introduced in this paper, taken the casting valve body for an example. The melt silicon rubber is injected in the valve body to duplicate its internal cavity surface, which is transformed into the external surface of the concretionary silicon rubber entity. The technical hurdle of collecting internal cavity surface data is solved. The key modeling technologies are studied according to data of spot cloud collected by laser-scanning approach, data preprocessing of spot cloud based on synthetic method, respectively constructing contour curves of internal and external piping surfaces, building the sold models corresponding internal and external surfaces on basis of alignment of their coordinates. The final CAD model of valve body is created after Boolean calculation of two sold models. This technical process is be efficiently used to solve the bottleneck of the rapid design and lean development about part with internal cavity structure.