The R-test measuring instrument is a special device for measuring the structure error of the rotating axis of five-axis Computer Numerical Control (CNC) machine tools. Its structure and parameters determine the measurement range and reliability. The typical structure and measurement principle of the non-contact R-test measuring instrument with eddy current displacement sensor was analyzed. Under the condition that other components were determined, by optimizing the structural parameters of the R-test measuring instrument-the sensor elevation angle α and the sensor center distance λ, the sensitivity and the measurement space are maximized, and the performance of the measuring instrument is effectively improved.