In order to solve the problem of aging and damage in the electric parts, slow data processing and the less parameters evaluation of Talyrond 73 Roundness instrument, a simple and feasible scheme of reforming the software and hardware of roundness instrument is put forward on keeping it in mechanical structure and the hardware precision. A new measuring electrical system and control system are developed and two kinds of methods on sampling signals are implemented. The software is including roundness measurement, spindle error separation, filtering, the graphic display, printing and data storage. Then the multiple parameters measurement and output are successfully implemented. The accuracy and stability of the instrument are improved by the new electric control system.The Specifications are better than the requirements of class one in JJG429-2000 Measurement Standard Instrument of Roundness and Cylindricity.