Traditional measuring methods of geometric parameters of asphere cannot meet the accuracy requirements as the aperture and vertex radius of curvature increase. In order to realize high accuracy measurement of geometric parameters, analysis and measuring methods are studied according to the null lens testing. The theoretical relationship between radius error and conic constant error is compared with the simulation results, then the requirements of measurement and data processing check method are determined. The basic principles of using laser tracker to measure geometric parameters are presented. The feature transformation, modeling and data processing methods are introduced. Experimental results show that the repeating precision of radius, conic constant and off-axis distance are ±0.039 mm, ±2.05×10-5 and ±0.055 mm, respectively. The high repeating accuracy indicates that this method is suitable for geometric parameters measurements, especially for large aperture and long radius aspheres. The absolute accuracy needs to be further analyzed.