Method for calculating function size of sphere part

A method for calculating function size of a sphere part comprises the steps of firstly measuring and obtaining coordinates of measured points on a measured spherical surface, then providing primary iteration parameters of the sphere, inquiring measured points in contact with a sphericity error tolerance zone, confirming the direction vector and the change amount of translation change of the tolerance zone according to the number and coordinates of contact points under the condition that the contact points are effectively in contact, calculating coordinates of changed sphere center, then inquiring the contact points again to perform the translation change again, sequentially performing iterative computation till criterions are met, and outputting a sphericity error and a sphere parameter optimal value. By means of the method, the sphere radius and a sphericity error are calculated, function surface criterions are met, and the method is an optimal solution.