This paper presents a scanning squareness measurement method for large ultra-precision components. A rectangular block as the squareness reference is used. The squareness error of the rectangular block is eliminated according to the geometric principle that the sum of four internal angles of a rectangle is 360°. And the straightness error of each line of the rectangular block is eliminated by means of scanning method with two 1D probes. The above two error separation technologies are combined effectively and the data processing method is developed. Additionally, the standard uncertainties including tilt and squareness errors of the rectangular block, temperature drift and random errors of the measured values of the probes are analyzed theoretically. It is confirmed that a combined standard uncertainty of less than 1 arcsec can be obtained for typical values of the parameters.