Rotary precision of the lathe spindle directly affects accuracy of the machined work-piece. With continuous improvement on requirements of rotary precision of the lathe spindle, a more accurate measurement method is needed to adopt to reflect actual shape precision of unprocessed part. Some advanced measurement principle and methods about rotary precision of the lathe spindle are researched, and also the measurement error is analyzed. The measurement result gotten by sensor concludes two parts, one is eccentric error caused by installation of the base ball, and the other is rotary error of the lathe spindle. The eccentric error is separated by the way of filter, as well as the rotary error is quickly and correctly measured by unidirectional digital measurement. The rotary error is superimposed on a base circle generated by algorithm. The roundness error is assessed by MZC. At last, verifying the feasibility of the new measurement method by means of real experiment.