Method for measuring plane size based on substitution method

The invention relates to a method for measuring a plane size based on a substitution method. According to the method for measuring a plane size, based on a known theoretical design distance X0 betweena first measurement point A and a second measurement point B of a product to be measured, the product to be measured is placed in a coordinate system of a measurement system, the coordinate system isdivided into several consecutive unit measurement areas with unit values of D, and the relative coordinate values of boundaries of the unit measurement areas where A and B are located are measured. According to a formula L=Delta X1-Delta X2+X, the X, the theoretical design distance X0 and X0+/-D are substituted into the formula for calculation, three measurement values are obtained, then the three measurement values are compared with the theoretical design distance X0, and a measurement value closest to the theoretical design distance X0 is taken as an actual distance between the measurement point A and the second mea