Preparation method of polymer

The invention provides a preparation method of a polymer. The preparation method comprises the following steps: 1) providing a reactant, wherein the reactant at least comprises a first compound, a second compound and a radical initiator; the first compound is shown in a formula I; the second compound is shown in a formula II; the first compound is provided in the form of a water solution or an ethanol water solution; the second compound is provided in the form of an ethanol solution; 2) mixing the reactant to obtain a mixture; 3) heating to carry out reaction; 4) after the reaction is completed, continuously heating to evaporate solvents and preparing the polymer through vacuum drying. The preparation method has the beneficial effects that a (meth)acryloyloxy ethyl trimethyl ammonium chloride water solution sold on the market is directly utilized as the reactant to undergo reaction and a solid is unnecessary to be extracted from the water solution and dissolved in organic solvents and surfactants are unnecessary to be used, thus greatly reducing the production cost; the reaction product is more applicable as a functional carrier of medicines.