Combining the non-diffracting beam datum-line method with the roundness measuring technology, a high precision coaxiality measurement method for a hole whose diameter is more than 40mm is proposed. The principle of the measuring system is introduced and error analysis and test to verify it are presented. The measuring uncertainty of this coaxiality system is less than 1µm when the measuring range is within 1.5m. The measuring uncertainty of error of roundness is less than 0.4µm when the error measuring range is -200~+200µm. This system has many advantages and it can be applied widely for coaxiality measurement.