To analyze test errors induced from system noise of swing arm profilometer (SAP) testing large aperture off-axis aspherics with different scan arcs, based on the Monte Carlo method, the simulation test model was built with different scan patterns from 8 arcs to 120 arcs. To evaluate the test sensitivity, test errors induced from system noise was fitted by Zernike polynomial. The low order test errors were reduced rapidly as scan arcs increased from 8 arcs to 39 arcs, and reduced slowly from 40 arcs to 70 arcs, and kept almost the same from 71 arcs to 120 arcs. As an experiment, a Φ1 500 mm off-axis aspheric mirror was tested by SAP scanning in 36 arcs, 72 arcs and 96 arcs respectively. The surface test error of 36 arcs are much larger with result of 7.73 μm in Peak-Valley (PV) and 0.68 μm in Root-Mean-Square(RMS). The surface test error of 72 arcs and 96 arcs are much close with result of 5.755 μm PV, 0.568 μm RMS and 5.612 μm PV, 0.569 μm RMS respectively. So the simulation results were verified and provided a guide for the scan line selection for SAP testing large off-axis aspherics.