Abstract: The neutron diffraction technique tests residual stress by comparing the crystalline lattice spacing of the test sample with that of a stress-free reference sample. Consequently, the stress-free reference is an important part of neutron residual stress characterization, and its preparation and testing directly affect the accuracy and error of obtained residual stress. In the present work, the preparation and testing methods of stress-free reference samples were systematically introduced based on neutron residual stress experiment cases. As stress-free reference, the applicable conditions, calculation methods, advantages and disadvantages of powder samples, cube samples, comb samples, plane stress assumption and slice samples were illustrated in details. The stress-free reference is an important part of neutron residual stress characterization, and its accuracy directly affects the test results of residual stress. It is necessary to select a reasonable stress-free reference preparation and testing method based on the composition, process, and size of the sample. This work provides both theoretical guidance and practical instructions for users aiming to prepare the stress-free reference samples and precisely test the deep three-dimensional residual stress in engineering components.