Abstract: Fractal theory is applied to study the shape characteristics of mined-out area in metal mines.The fractal dimensions of four different single mined-out areas in metal mines have been calculated in one-dimensional lines, two-dimensional areas, three-dimensional curved surface area and body, and the results show that the shape of mined-out area has good fractal characteristics.Then, the method had been extended to the fractal dimension calculation of the 3D surface roof of mined-out area group, it was proved that the 3D surface roof of the mined-out area group has obvious fractal characteristics too.Fractal dimension is a quantitative parameter that reflects the complexity of mined-out area, it provides a new method for the study of complex mined-out areas, and it can be used for reference of similar study.