The blade configuration and drawing of pump impeller has since long attracted the focused efforts for the computer aided pump auto-design. For this purpose, the configuration of flow passages and streamlines in axial plane undoubtedly becomes the key process to be solved firstly. The available references show that there are some shortcomings in almost all current methods, among which the utilization of manual design thought is the main problem. For the mentioned reason, a new method of determining flow passage and streamlines in axial plane is proposed for the pump's impeller design in the paper, which utilizes the PDE (Partial Deferential Equation) numerical grid generation techniques of the CFD pre-process method. The main idea of the presented method is that two clusters of orthogonal curves can be used as the representation of streamlines and across streamlines respectively. Then it would be feasible to generate two clusters of orthogonal grid lines through the numerical solution of partial differential equations, i.e. Laplace Equations. In this method, the formation of the grid distribution of axial region depends entirely on the geometry of the configuration lines of the pump's front and back covers' shape and is irrelevant to its installed position and properties of fluid, hence the method has geometrical invariability in comparison with the manual design. The results show that the presented method possesses advantages of more definitive theoretical background, high precision, small amount of computation and ease in programming.