The present invention discloses a multi-arm polycarboxylate water reducer and a preparation method thereof, the preparation method includes the following steps: making a phenolic hydroxyl containing rigid compound and p-nitrobenzonitrile subjected to a nucleophilic substitution reaction in a solvent, then subjected to addition with a sodium azide in a solvent and water mixed solution to obtain a tetrazole derivative; making an unsaturated polyether macromonomer subjected to terminal halogenation and react with the prepared tetrazole derivative to obtain a tetrazole-containing multi-arm unsaturated polyether macromonomer; and subjecting the tetrazole-containing multi-arm unsaturated polyether macromonomer, an unsaturated carboxylic acid small monomer and an unsaturated polyether macromonomer to a free radical polymerization reaction under combined action of an initiator, a reducing agent and a molecular weight regulator to obtain the multi-arm polycarboxylate water reducer integrating shrinkage reducing and antibacterial functions.