An ultra-precision machine is developed by Precision Engineering Research Institute of Harbin Institute of Technology to machine components made of KDP crystal with single point diamond fly cutting technique . A stable ultra-low speed feed of worktable is necessary in the machining process inasmuch as the KDP crystal components to be machined must be high form accuracy, low surface roughness and low surface waveness . This paper analyses the effect on speed stability and positioning accuracy under the control of semi-closed loop and full-closed loop based on the experimental data, and also present a compensating control algorithm of error disturbance feed-forward which enhances the stability of ultra-low speed motion of the semi-closed loop feed control servo system of the machine. The simulation results indicate that the values of the steadystate tracking error decreased to 1/10 after using compensating control algorithm. The P-V value of the aluminum specimen machined by the ultra-precision machine tool was 0.27 wavelengths.