Abstract: Combined the extended discharge design theory with similar-design method, this paper developed a more perfect pump design method, named variable-condition similar-design method. This method especially suites CAD system for some types of pump, and it is easy to be realized with CAD. It has a great difference from the traditional similar-design method, i. e. .taking a special operating point instead of the optimum one as a calculating point. The special pcint is selected by a Certain procedure. To show the capability of the method, the process of design of a centrifugal pump is described in details.