Plenty of design parameters are used during electric machine design process. Generally, design parameters are looked up from existing curves drawn by magnetic field graphing method or analytical method. Affected by different draftsmen's proficiency, precision of parameter curves drawn by magnetic field graphing method have uncertainty. Using analytical drawing method to draw design parameter curves usually simplifies analytical model of electric machine to improve calculating efficiency. But simplified analysis model results in more or less accuracy problems. In order to improve the accuracy of electric machines design calculation, finite element method (FEM) is used to analyze electromagnetic field and calculate design parameters based on complete electric machine model in this paper. As the analytic objects are a series of 300MW evaporative cooled generator-motor, design parameter curves drawn in this paper should be more accurate than existing curves for electric machine with similar size and structure.