The invention relates to a preparation method of hydrogel and aims at providing a preparation method of novel double-network hydrogel with high water absorption and high strength. The preparation method comprises the following steps: firstly, a first network monomer, a cross-linking agent, a dispersion medium and an initiator are dissolved in water, and a solution is poured into a silica gel sheet mould, irradiated under strong ultraviolet radiation, stood and cooled; gel is taken out of the mould, put into a water solution containing a second network monomer, a cross-linking agent and an initiator, swollen until balancing, and then taken out to carry out weak ultraviolet radiation so as to obtain the double-network hydrogel with high water absorption and high strength. The invention has simple preparation process and can quickly obtain the double-network hydrogel with superior performance and high water absorptivity. The hydrogel has very good water absorbing capacity and superior mechanical property and can bear external force well. The hydrogel can be widely applied to the field of biomedicine comprising medicine wrapping, medicine slow-controlled release, artificial eye cornea, burn protective films and articular cartilage restoration and replacement and also has potential application value in the field of the light industry.