The present disclosure provides a method for preparing plastic composites containing a plant fiber. The method includes providing a first raw material containing the plant fiber and a second raw material containing plastics; pretreating the first raw material; heating the pretreated first raw material for carbonization; and mixing the carbonized first raw material with the second raw material to form a composite material. The first raw material may be coffee grounds, and the pretreatment of coffee grounds includes: washing the first raw material with water to remove contaminants and water-soluble compounds, drying the washed first raw material, extracting the oily substance from the dried first raw material using a solvent, obtaining the extracted oily substance from the first raw material; and completely removing the solvent from the extracted first raw material. The plastic composites having coffee grounds prepared in accordance with the method of the present disclosure has improved mechanical properties, tensile properties and thermal stability, and has a wider range of applications.