Preparation method of residue-free dual-network weighted fracturing fluid thickening agent

The invention discloses a residue-free dual-network weighted fracturing fluid thickening agent which is prepared by reacting a nonionic monomer, a flexible polymer, a cross-linking agent and an initiator in a solvent deionized water. The flexible polymer is at least one of isotridecanol polyoxyethylene ether, fatty alcohol polyoxyethylene ether or methyl allyl polyoxyethylene ether. The nonionic monomer is acrylamide or N-(4-aminophenyl) acrylamide. The preparation method of the thickening agent comprises the following steps: (1) adding the deionized water, the nonionic monomer, the flexible polymer and the cross-linking agent into a reaction kettle, and introducing nitrogen to remove oxygen; (2) dissolving an initiator in deionized water, and introducing nitrogen to remove oxygen; and (3) adding the initiator solution into the reaction kettle, stirring, putting into a water bath, standing at 40 DEG C, and preserving heat for 8 hours to obtain the target thickening agent. Fracturing fluid formed after crossli