1. A overlay measurement method, comprising: providing three predetermined patterns: a first predetermined pattern, a second predetermined pattern and a third predetermined pattern; performing an inspection process on said three predetermined patterns, to obtain three image points: a first image point, a second image point and a third image point respectively; performing a defining process to define a default position; and performing a calculating process to obtain a real offset value x=(p−q)*(c−a)/(a−b)+p, wherein: p is the distance between the first predetermined pattern and the third predetermined pattern; q is the distance between the second predetermined pattern and the third predetermined pattern; a is the offset value between the default position and the first image point; b is the offset value between the default position and the second image point; and c is the offset value between the default position and the third image point; wherein said inspection process, said defining process and said calculating process are performed through a computer system.