The invention provides a crosslinked polymer, a hydrogel, a water-based fracturing fluid comprising the same, and methods of making and using thereof. The crosslinked polymer of the invention is represented by formula (I), wherein * denotes a combining site with a polymer starting material and *′ denotes an optional combining site with the polymer starting material, wherein the combining sites denoted by * and *′ may be located in the same polymer molecule, or in different polymer molecules, but there are at least two combining sites located in different polymer molecules; X1 and X2, which may be the same or different, are independently an oxy (—O—) or imino (—NH—) group; X3 is an oxy (—O—) or imino (—NH—) group when *′ denotes a combining site with the polymer starting material, or X3 is a halogen, NH2 or OH when *′ isn't a combining site with the polymer starting material. The crosslinked polymer of the invention can be used in water-based fracturing fluid, in personal care products, household care products and pet care products, in reusable hydraulic fracturing flowback fluid, in gel explosives or in fire extinguishing mortar.