Abstract: The additional friction moment of ball screw as a result of preload is constituent part of the driving moment of screw drive system. It is pointed out by means of analysis the acting force between two screw nuts is varied obvious along with axial load, so the additional friction moment is also, that is should taken into account in accuracy analysis and calculation, but it is not indeterminate and also there are not correlate calculation methods in currently conclusion about that. For this purpose the variety rule and solution of interaction acting force between two screw nuts are infered out, and then the new calculation formula and variety ratio representation of additional friction moment are infered out. The variety rule of additional friction moment and driving moment along with axial load are discussed moreover, and also confirmation principles of additional friction moment in checking process of total moment are discussed. It is verified through experiments that the new calculation method and conclusion are reasonable.