In order to foresee the influence of random processing errors on geometric curve in design stage, meanwhile including success and failure process during the gradual change process in the forecast. Based on probabilistic fuzzy reliability point of view, the success or failure determination will be extended to fuzzy events. The geometric curve deign method will be also given when taking the impact of random engineering error into account. Related analysis formulae and the fuzzy criterion of success or failure of designing the curve process are established and derived. Through which, design and engineering process are integrated, the designer will be more reliably to predict the success or failure of the geometric curve design during the design stage. The processing error of lack of statistical data and the objectivity of the success or failure determination criterion will be easily solved. Economy cost and reliability design of geometrical curve design will be also considered.