1. An operating method for a design system associated with prestressed concrete cylinder pipe (PCCP) using a graphic user interface (GUI), comprising the steps of: a) displaying an initial window for a PCCP design when a PCCP icon is selected with a correct password being input; b) determining whether or not there is an icon selected from the initial window containing icons respectively associated with an embedded cylinder pipe (ECP) design, a lined cylinder pipe (LCP) design, a fitting design, a computer-aided drawing (CAD) operation, a quantity calculation, an inquiry to a Database, and a demo presentation; c) displaying a full-screen input window for an ECP/LCP design program when the ECP/LCP icon is selected at said step (b), and inputting design data, associated with a pipe to be designed, including pipe data, material property data, and design load and pressure data; d) executing a calculation for the structure of the pipe to be designed, based on the input data in accordance with the ECP/LCP design program, and displaying a full-screen output window; and e) determining whether or not the user satisfies the design result on the output window, and returning to said step .COPYRGT. when it is determined that the user does not satisfy the design result, while storing output data, indicative of the design result, in the Database when it is determined that the user satisfies the design result.