Combing with the open structure of the numeral control system and using the basic motion function of the motion control card, a control software for the three axis engraving machine was developed based on the tools of VC++. In order to improve the smooth performance of speed on the inflection, the nonlinear relation between inflection speed and acceleration was built based on the improved model of the hyperbolic tangent function. The actual engraving experience proves the method has the character of the reliability and stability.