In this paper, a virtual breast plastic surgery planning method is proposed, which reconstructs the breast after excision for certain diseases such as cancer. In order to achieve a rational result, we calculate shape, area, volume and depth of the skin and muscle for the reconstruction, based on the other healthy breast. The steps are as follows: 1) input breast's MRI data of patient; 2) get the healthy breast using balloon segmentation algorithm and get triangle mesh on breast surface; 3) flatten the triangulated skin of breast using deformable model to attain the shape and volume of the flap for breast reconstruction. Other methods such as mesh smoothing and cutting of triangulated surface are also introduced. The doctors validation and evaluation process are also provided to ensure the robust and stable result of virtual surgery planning.