The invention provides a method for irradiation-chemical method combined treatment of plant fibrous agricultural wastes. The method comprises the following steps: pretreating the plant fibrous agricultural wastes, then carrying out dynamic irradiation and crushing treatment, weighing treated wastes, pouring the treated wastes into a reactor, adding a solvent, carrying out a reaction, and carryingout vacuum filtering so as to obtain a filtrate A and a filter residue, subjecting the filter residue to washing and drying so as to obtain cellulose, then subjecting the filtrate A to reduced pressure concentration, adding distilled water into a concentrate so as to extract a lignin precipitate, carrying out filtering so as to obtain a filtrate B and a filter residue, subjecting the filter residue to washing and drying so as to obtain lignin, subjecting the filtrate B to reduced pressure concentration, and carrying out collection so as to obtain xylose. The invention also provides a system for irradiation-chemical method combined treatment of the plant fibrous agricultural wastes. The method provided by the invention uses common and easily-available reagents like formic acid in all steps,has low cost, and facilitates large-scale popularization and application; and compared with the prior art, the method improves production efficiency, and is a green and environment-friendly treatmentmethod.