The invention discloses a thickening agent which is provided with a branched chain structure and used for water base fracture. The thickening agent is composed of six monomers in a copolymerization mode. The six monomers comprise N, N-dimethyl acrylic amide (AAM), dimethyl diallyl ammonium chloride (DADMAC), acrylonitrile (AN), methacrylic acid (AA), 2-acrylic amide-2-methyl propane sulfonate (AMPS) and N-VINYL-2-pyrrolidone (PVN). Further disclosed is a preparation method of the thickening agent with the branched chain structure. Compared with the prior art, the thickening agent has the advantages of being low in residue, temperature resistant, prone to dissolution, anti-shearing and good in sand carrying property. The hyperbranched molecule fracturing fluid thickening agent completely solves the problems that vegetable glue fracturing fluid is high in damage and friction, general polymer fracturing liquid is not resistant to shear and incomplete in gel breaking, and the like. The thickening agent is not like vegetable glue which contains a large amount of water insoluble matters, completely dissolved in a fracturing liquid stock solution, basic zero in the water insoluble matters and free of containing solid phase composition. The obtained fracturing liquid stock solution is glittering, translucent and bright. After gel is broken, compared with a vegetable glue fracturing liquid system, contend of residue is greatly reduced. The thickening agent has the advantages of being less in residue content, low in use viscosity, good in sand carrying property, low in cost, convenient to flow back and the like.