On the basis of a brief description of the measurement principle of a 3.39 μn dual wavelength beat wave interferometer, a new method of aiming and positioning the measuring points of a large dimension workpiece diameter is put forward. The high precise absolute measurement of a large diameter without a guide is realized by using a 3.39 μm dual wavelength beat wave interferometer. The results of aiming and positioning experiment are given. The measurement error of the system is analysed. The error analysis indicates that the uncertainty (3σ) in measuring a diameter of 10 meters is less than 0.03 mm.