Summary: In order to confirm the function value more precisely and quickly from a kind of function that was provided in the form of curve graph, an algorithm was put forward on numerical value method. Based on the provided function values of the points that are in the curve, the algorithm confirmed the function value of any point by several times Lagrange interpolation. Furthermore, the algorithm is easily carried out by computer program. In order to compare the algorithm put forward in this paper with the traditional one, a certain practical example was calculated by the two algorithms respectively. The result is that the average relative error of the algorithm put forward in this paper is 2. 1%. This indicates that the algorithm put forward in this paper is better than the existing engineering method.