A method for obtaining projected outline of tumour3-D dissection structure in radiotherapy system includes projecting tumour3-D dissection structure onto projection plane to set up label image, mapping projection line section obtained from projection plane on said label image to obtain binary label image by drawing lines on label image, carrying out scan-search on pixel point of binary label image according to specified direction to confirm initial outline then tracking each outline point to obtain projection outline of tumour 3-D dissection structure.