The experiment on slender shaft open-cycle belt grinding process is conducted in this paper. The research objects are dimensional accuracy and deviation from roundness error, the changes of which are emphasized after the belt grinding. And the factors affecting the working accuracy of the belt grinding are analyzed. Some measures for improving working accuracy of the belt grinding are put forward. Research result shows that by installing a belt grinding device on a lathe to grind the slender shafts can improve the accuracy with high efficiency.