A preparation method for and an application of a modified siloxane-based supercritical carbon dioxide thickener are disclosed. The method includes the following steps: using 1,3-diallyl-1,3,5-triazine-2,4,6-trione as a polymerization monomer, and dropwise adding 1,3,5,7-tetramethylcyclotetrasiloxane for hydrosilylation in the presence of a catalyst to obtain a modified siloxane polymer. The obtained modified siloxane polymer is directly used as a supercritical carbon dioxide thickener without purification post-treatment; when in use, the modified siloxane polymer is prepared into a solution with a concentration of 1-2%, has an excellent thickening effect with no cosolvent required, and has high solubility in carbon dioxide and high temperature resistance.