This paper studies the effect of measuring angle error in X-ray powder diffractometer, which is caused by diffractometer inherent angle scale error resulting from mechanical manufacture, on the precision of calculated lattice parameter. It represents the theoretical limit of the consistency of the lattice parameters obtained by different diffractometers and laboratories. We use the calculated polysilicon diffraction patterns with random angle error to simulate the results measured by many sets of diffractometers of some manufacturing precision, then calculate and analyze the lattice parameters by three methods.