Abstract: In order to investigate the seawater corrosion resistance of high-density polyethylene (HDPE) based wood plastic composites, four types of plant fibers (eucalyptus, poplar, bamboo, and rice husk fibers) were incorporated into HDPE. The simulated accelerated seawater corrosion test was carried on to analysis the mechanical properties, color stability, microstructure and chemical groups of four types of plant fibers/HDPE composites before and after corrosion. The results show that the accelerated seawater corrosion would result in poorer fiber/matrix bonding quality (increasing cracks and holes), mechanical properties and color stability (the color shifts towards white, yellow and green for eucalyptus/HDPE and poplar/HDPE and rice husk/HDPE composites, and the color shifts towards white, blue and green for bamboo/HDPE composites), and higher hydroxyl content. After 21 days seawater corrosion, the reductions of the flexural strength and modulus of eucalyptus/HDPE composites are 12.94% and 23.18%, respectively; for the bamboo/HDPE composites, the reductions are 15.45% and 23.20%, respectively; for the rice husk/HDPE composites, the reductions are 18.53% and 25.15%, respectively; and for the poplar/HDPE composites, the reductions are 18.52% and 34.21%, respectively. The eucalyptus/HDPE and poplar/HDPE composites present the lowest and highest degradation (such as mechanical loss, color fading and interfacial debonding) after corrosion, respectively.