Abstract: To reduce costs, increase environmental friendliness, and obtain a good wood feeling, ultra-highly filled polypropylene-based wood plastic composites (UH-WPCs) were successfully prepared by extrusion technology using poplar wood fiber and bamboo fiber as raw materials. Based on pronounced reduction in polypropylene matrix content, the effect of filling content and fiber species on mechanical properties and creep resistance at high and low temperatures, thermal expansion, dimensional stability, and water uptake behavior of the UH-WPCs were investigated. The results show that as the filling content increases from 75wt% to 90wt%, the linear thermal expansion coefficient of the UH-WPCs decreases drastically, creep strain decreases gradually but increases at 90wt% filling content, and the tensile and flexural moduli increase firstly and then decrease at 90wt% filling content. The tensile strength, flexural strength and impact strength decrease gradually with the increasing filling content. The UH-WPCs show higher tensile and flexural properties at −30℃, while the impact toughness is higher at 60℃. The variation of temperature, humidity and moisture content can lead to the dimensional change of UH-WPCs. The UH-WPCs exhibite obvious anisotropy in dimensional change. The largest change in dimension was observed in the thickness direction, followed by width and length direction. The effect of humidity on the dimensional stability of UH-WPCs is more significant than that of temperature. The UH-WPCs prepared with wood fiber show better comprehensive properties than that with bamboo fiber, which is attributed to the larger aspect ratio of wood fiber and better interface adhesion of wood fiber-filled UH-WPCs. The research of UH-WPCs provides a theoretical basis for reducing cost of WPCs and broadening its application scenarios.