High-entropy alloys (HEAs), characterized by "multi-principal elements", possess exceptional mechanical properties compared with conventional alloys based on "a single-principal element". However, owing to this “multi-principal element” nature, high-entropy alloys exhibit complex deformation behaviors dominated by alternating and coupled deformation mechanisms. Therefore, elucidating these intricate deformation mechanisms remains a key challenge in current research. Neutron diffraction techniques offer distinct advantages over traditional microscopic methods for characterizing such complex deformation behaviors. The strong penetration capability of neutrons enables in situ, real-time, and non-destructive detection of structural evolution in bulk samples under complex environments, and neutron diffraction allows precise characterization of lattice site occupations for light elements (e.g., C, O) and neighboring elements. This review begins with the principles, experimental procedures, and data analysis of neutron diffraction, combines them with recent advances in face-centered cubic high-entropy alloy research, summarizes and presents typical examples of using neutron diffraction to investigate their deformation behaviors, ultimately revealing deformation mechanisms dominated by dislocations, stacking faults, twinning, and phase transformations.