Computing method of function sizes of benchmark constrained cylinder

The invention discloses a computing method of function sizes of a benchmark constrained cylinder. The computing method of the function sizes of the benchmark constrained cylinder comprises measuring and obtaining coordinates of measuring points which are arranged on a cylinder to be measured and a benchmark; fitting the benchmark based on a minimum area to provide a primary parameter of the cylinder to be measured, projecting the measuring points in a plane which is perpendicular to an axis of the cylinder to be measured and fitting the projecting points through a method of computing a function surface of a circle; inquiring a measuring point the cylinder to be measured and the function surface are in contact with and performing corresponding rotating change or size change on the function surface according to coordinates of a contact point; and inquiring the contact point to perform change of the function surface, sequentially performing iterative computations until a discriminate criterion is satisfied and outputting a parameter of the cylinder to be measured.