The direct substitution method is used in the laser beam widths meter (usually the laser beam profiler) calibration. The laser beam widths measurement results of device under test (DUT) are compared with those made by the laboratory standard. For most laser beam widths meter, the detector measurement plane is installed inside detector head, so it is difficult to accurately position the measurement plane for both standard and DUT. Depending on the ocular estimation, the difference of measurement plane between standard and DUT is usually controlled within 5mm. For 2.5mrad divergence angle of laser beam, 5mm tolerance will cause an uncertainty of 12.5μm in the beam widths measurement. In the optimized calibration system, a wedge prism placed in the beam path produces a precise beam deflection, which results in the laser spot displacement on the measurement plane. Depending on the measurable displacement of beam centroid position and the beam deflection angle, the distance from the wedge prism to the detector measurement plane is determined. The experimental results show that using the measurement plane positioning method, the difference of measurement plane between standard and DUT is close to 80μm, only contributing an uncertainty of 0.125μm in final result with the same 2.5mrad beam divergence angle. In additional, some factors affecting precisely positioning of measurement plane are analyzed in this paper.