A method for preparing composite including coffee grounds comprises: drying 20 ∼ 50wt% of coffee grounds such that a water content of the coffee grounds is 0.1% ∼ 3%; stirring and mixing the dried coffee grounds with 40 ∼ 70wt% of plastic, 5 ∼ 25wt% of calcium carbonate and 3 ∼ 10wt% of mix modifier comprising 1.8 ∼ 6wt% of a coupling agent and 1.2 ∼ 4wt% of a binder; and granulating the stirred and mixed materials so as to form a first granular composite, and then smashing the first granular composite to become a second granular composite for export and collection; wherein the coffee grounds is dried by a drying device such that the water content of the coffee grounds is 0.1% ∼ 3%, further comprises delivering the dried coffee grounds to a screening and filtering device to filter unnecessary impurity before further delivering the dried coffee grounds, the plastic, the calcium carbonate and the mix modifier into a high-speed blending device by a feeding device, stirring the dried coffee grounds ()71, the plastic, the calcium carbonate and the mix modifier using the high-speed blending device, and when stirring and mixing performing additional treatments of venting water vapor and heating by increasing the temperature to 160°C, and decreasing the water content of a resulting mixed material to 3%, wherein the mixed material is stirred by a main granulating device and is exported as a first granular composite, the first granular composite is cooled by a discharging and assembling device, the cooled granular composite is smashed to become a second granular composite, and the second granular composite is exported for collection. drying 20 ∼ 50wt% of coffee grounds such that a water content of the coffee grounds is 0.1% ∼ 3%; stirring and mixing the dried coffee grounds with 40 ∼ 70wt% of plastic, 5 ∼ 25wt% of calcium carbonate and 3 ∼ 10wt% of mix modifier comprising 1.8 ∼ 6wt% of a coupling agent and 1.2 ∼ 4wt% of a binder; and granulating the stirred and mixed materials so as to form a first granular composite, and then smashing the first granular composite to become a second granular composite for export and collection; wherein the coffee grounds is dried by a drying device such that the water content of the coffee grounds is 0.1% ∼ 3%, further comprises delivering the dried coffee grounds to a screening and filtering device to filter unnecessary impurity before further delivering the dried coffee grounds, the plastic, the calcium carbonate and the mix modifier into a high-speed blending device by a feeding device, stirring the dried coffee grounds ()71, the plastic, the calcium carbonate and the mix modifier using the high-speed blending device, and when stirring and mixing performing additional treatments of venting water vapor and heating by increasing the temperature to 160°C, and decreasing the water content of a resulting mixed material to 3%, wherein the mixed material is stirred by a main granulating device and is exported as a first granular composite, the first granular composite is cooled by a discharging and assembling device, the cooled granular composite is smashed to become a second granular composite, and the second granular composite is exported for collection.