The performance of high-density polyethylene (PE) pipes joints directly affects the total pipeline’s operation, and so studying the residual stress of butt fusion joints is crucial for enhancing the safety of gas pipelines. Based on a layer-by-layer ring cutting test method, we measured the distribution of residual stresses in the fusion zone and heat-affected zone of butt fusion joints for PE gas pipes. Firstly, the ring samples were cut, their diameter changes were measured, and the results were compared with those predicted by the theoretical calculations. This showed that the circumferential residual stresses of the butt fusion joint for the PE gas pipes are exponentially distributed in the base material (BM) zone, the weld zone (WZ) and the heat-affected zone (HAZ). Furthermore, the residual stresses in the HAZ are lower than those in the BM zone, and the smallest residual stresses were seen in the WZ. Finally, using X-ray diffraction (XRD) technology, the crystallinities in the BM zone, the WZ, and the HAZ of the butt joints were measured. The crystallinity gradually decreased from the WZ to the HAZ and the BM zone, and the crystallinity in each zone was also related to the magnitude of the residual stresses.