Through the establishment of symmetric dual-tools turning model of slender shaft, the theoretic relieving amount formulas of cutting tool machining point were derived. The relieving amount data of the cutting tool point was gotten based on the static simulation analysis about the dual-tools turning of slender shaft. The correction coefficient k was introduced in the theoretic relieving amount formulas of cutting tool machining point, by considering the relationship between cutter radial force and back engagement of the cutting edge, as well as the tool relieving amount of error due to other factors. The value of the correction coefficient k was determined through comparing analysis of the simulation curve and forecasted theoretical curve. Finally the numerical control turning experiment was carried out. The results show that let the optimized theoretic relieving amount formulas of cutting tool as the compensation function of turning a slender shaft can significantly improve the machining accuracy.