The invention provides a preparation method for new type fully-degradable cellulose composite fiber with skin-core structures. The preparation method is characterized by comprising steps: cellulose and biomass polyesters are subjected to vacuum drying, the cellulose and ionic liquids are added in a high-speed mixer according to the ratio and stirred evenly, the mixture of the cellulose and the ionic liquids is added in a co-rotating twin-screw extruder through a feeding inlet, the biomass polyesters obtained from the first step are added through another feeding inlet, both of the mixture and the biomass polyesters are extruded by the co-rotating twin-screw extruder into a spinning pack to be spun through spinneret plates, the obtained composite fiber goes through a water tank and the ionic liquids in skin layers are washed off, then the composite fiber is stretched and wound through the spinning pack, and the new-type fully-degradable cellulose composite fiber with skin-core structures is obtained. The composite fiber has advantages of good hygroscopicity, comfortableness and chromaticity of cellulose and good mechanical properties of polylactic acid, and retains the biodegradable characters of cellulose and polylactic acid, and is suitable for the medical and taking fields.