For the measurement of large SiC mirror before interferometry test, the method of swing arm profilometry (SAP) is introduced. The basic principle and flow chart of SAP are presented; with surface reconstructed based on consistency of surface height at scan crossings and defocusing amount of mirror surface measured with a laser tracker as supplementary, an optimized test model is built. As an example, a 2040 mm SiC mirror is tested by SAP, with accuracy of 0.46 μm root mean square (RMS), which is 0.04 μm deviation from interferometric test. As a best-fitting sphere being basement of the test, in situ metrology is implemented; this method provides an accurate and efficient way to measure a large mirror, and meet the need of high-accuracy measurement of large SiC mirrors during grinding.