In motion systems driven by stepping motors, reference word interpolation is usually used with a constant period T, in which the existing position quantization errors will inevitably result in the trouble of velocity quantization errors. A separated-axis interpolator with variable period is proposed to deal with these troubles. The reassignment module of displacement and time is the key of the interpolator. In the module, the interpolation period of each axis can be separately adjusted upon its quantized displacement, thus resulting in almost ideal velocity profile and moving smoothness. Simulation and machining results are given and show that the proposed interpolator is effective in the improvement of motion stability and interpolation accuracy.