Swing arm profilometer is a useful metrology tool for large optics. For larger mirrors, the testing accuracy decreases as the arm becomes longer, while the testing accuracy requirement remains the same. We introduce a simple solution to make testing of large mirrors with shorter arms possible, which improves the testing accuracy, especially reduces the uncertainty of low-order shapes like astigmatism and trefoil. Simulation and experiment results show that testing uncertainty of low-order shape and high-frequency errors reaches 0.1 \mu m RMS, which faithfully meets the requirement of profile testing to guide the grinding and coarse polishing process.