Abstract: In this paper the integral routes of irrotational field are treated as rays, the potential values of the boundary points which be passed by the ray are treated as input and output, then Laplace Equation can be solved by inverse method. This method is innovatory and applicable, the results are precise and the boundary conditions are easy to be coped with. This is a useful numerical method for a kind of second-order partial differential equations that can be transformed to Laplace Equation.