An automatic circle grating dynamic testing instrument with high accuracy is presented in this paper- The expending uncertainty during dynamic measurement of circle grating is O. O8". A high rotation precision air static pressure bearing and multiprobe homogenization technique are adopted tothe instrument for decreasing the effect of the error of the reference ruled grating disc- In the meantime,the computer system is adopted to control the measuring process and process the data, thereby theautomatic measurement is realized. The result of test and measurement and the error curve are printedby the printer.