Abstract: The interfacial strain of wood fiber/high density polyethylene(WF/HDPE) composites was studied. Digital image correlation(DIC) was used to investigate the effects of WF mass fraction (10wt%–40wt%) and modified ammonium polyphosphate(mAPP) flame retardant mass fraction (10wt%–25wt%) on the strain distribution and transmission evolution of WF/HDPE composites. The mechanical properties and interfacial bonding of WF/HDPE composites were analyzed by mechanical tests and SEM, respectively. With WF mass fraction rising from 10wt% to 30wt%, the strain transfers stably and uniformly from both ends to the axial center of the WF/HDPE composite. When the WF amount reaches 30wt%, the high strain transfers within 1/2 region of WF/HDPE composite and its tensile strength and impact strength are 21.5 MPa and 10.22 kJ/m², respectively. However, when WF mass fraction is 40wt%, the stress concentration occurs at tensile bearing end of the WF/HDPE composite, and prevents uniform transmission of strain in WF/HDPE composites. mAPP exacerbates debonding and impedes mechanical meshing between WF and HDPE. As WF mass fraction increases from 10wt% to 25wt%, several scattered high strain regions appear and the full-field strain transfers irregularly. When the WF mass fraction reaches 25wt%, the strain distribution of WF/HDPE composite becomes polarized, resulting in a decrease of the tensile strength and impact strength to 15.5 MPa and 5.49 kJ/m², respectively.