Abstract: The stress triaxiality is the key factor to determine the evolution of metal damage. In order to understand the distribution of stress triaxiality in fine crystal structure, the microstructure information of pure copper produced by eight-pass equal channel angular pressing（ECAP） was obtained by electron backscattered diffraction（EBSD）. Based on the crystal plasticity finite element method, a representative volume element was established to investigate the distribution phenomena and laws of stress triaxiality in eight-pass ECAP pure copper under cyclic load. The results show that the distribution of stress triaxiality is uneven at the initial stage of cyclic load, and the inhomogeneity increases after 20 cycles of cyclic loading; the maximum and minimum stress triaxiality is mostly distributed at grain boundaries, usually in the case of soft and hard oriented grains adjacent to each other.