A plugging agent with temperature-resistant, salt-resistant and high-expansion for plugging large fractures and a preparation method thereof of the present disclosure are provided, the plugging agent system consists of the following components: main agent: 6%˜10%, cross-linking agent: 0.6%˜1%, initiator: 0.006%˜0.01%, additive: 6%˜14%, and the rest being water; the preparation method includes the following steps: adding montmorillonite to water to prepare a montmorillonite dispersion system; adding the main agent, the cross-linking agent and the initiator to the montmorillonite dispersion system to prepare a main agent solution; putting the main agent solution into an oven with a temperature range with 90˜20° C. for 9˜48 h to obtain gelation, the adhesive strength of the agent system can reach grade H, with 50˜80 expansion multiple, good strength after being expanded and good water plugging effect under the condition of a temperature of 140° C. and 250,000 degree of mineralization.