The invention discloses an efficient salt-resistant dual-purpose thickening agent, a preparation method and application thereof. The preparation method of the thickening agent comprises the followingsteps of: S1, preparing a water phase; S2, preparing an oil phase; S3, performing emulsifying; S4, conducting polymerization; and S5, carrying out aftertreatment. A hydrophobic monomer, a functional monomer and a salt-resistant monomer are introduced into the thickening agent to form a compact and complex spatial network structure in an aqueous solution, so that the thermal stability and the saltresistance of the thickening agent are improved, and the aqueous solution of the thickening agent can keep relatively high viscosity under a hypersalinity condition. Due to the addition of a chain extender, the molecular chain of the polymer is longer, and the viscosity is greatly improved. A cosolvent is introduced, so that the dissolving speed of a polymer emulsion is further increased, the polymer emulsion can be completely dissolved in 30 seconds, the requirement on water quality is low, and the polymer emulsion can be prepared from oilfield reinjection water and oilfield wastewater. Due to the addition of a suspending agent, the emulsion system is more stable, and the emulsion is not layered within two years. The thickening agent also has the characteristic of dual purposes, and can be used as a fracturing fluid slickwater drag reducer and a fracturing fluid thickening agent.