The invention discloses a method for preparing fluid gel by utilizing a formation temperature to form a blocking valve to block an air layer. The method comprises the following steps: a, preparing a stirrer; b, adding water into the stirrer, starting an impeller in the stirrer at normal temperature, and stirring the water; c, adding an initiation monomer and a cross-linking monomer while stirring the water, adding a gel initiator after the added initiation monomer and cross-linking monomer are dissolved, and continuously stirring to form gel blocking fluid; and d, pumping the gel blocking fluid into a shaft at projected depth to form fluid gel, wherein the water, the initiation monomer and the cross-linking monomer form a gel cross-linking system, the ratio of weight parts of the gel cross-linking system to the gel initiator is (10-30):1, and based on the total mass percentage of the formed fluid gel, the added initiation monomer accounts for 2-15 percent, the cross-linking monomer accounts for 0.1-5 percent, the gel initiator accounts for 0.1-2 percent, and the rest is water. The process is simple, and the prepared fluid gel is low in cost and has extremely high pressure resistance.