This study presents a development of DeCatS(Detail Design of High Speed Catenary System) which is a software to design high speed catenary system automatically. The program is developed by Korea Railroad Research Institute with the support of KHSR. A process of developing it and a comparison with LEXCAT. in order to demonstrate a preciseness of that, were performed. In the program, decision of H-beam, cantilever fitting, management of materials, automatic drawing and etc. according to input conditions can be accomplished. In this study, especially the program algorithm for the decision of mast was introduced.