According to Z-transform representations of discrete B-spline functions,the fitting and decomposing of the signal in B-spline space can be implemented with FIR filters and IIR filters.With combination of theory above and variational calculus,the B-spline filter was constructed to determine the mean line in surface roughness measurement.A zero phase shift filtering was used in the B-spline filter to obtain the difference equations.The experiment in surface profile of a workpiece validates this method.The B-spline filter also shows the characteristic of zero phase shift and its computing efficiency is three times higher than that of Gaussian filtering mean line.