The invention discloses a preparation method of a nanometer antibacterial agent for paper. The preparation method comprises the following steps: preparing cellulose modified with carboxyl, adding an AgNO3 solution into the cellulose modified with carboxyl, adjusting the pH value by using a sodium hydroxide solution or hydrochloric acid, stirring and mixing for a certain time, filtering and taking out the cellulose, performing high-pressure homogenization to obtain carboxylated modified nanometer cellulose which adsorbs Ag<+>, namely the nanometer antibacterial agent for the paper. The preparation method has the beneficial effects as follows: with the carboxylated modified nanometer cellulose as an Ag<+> carrier, the obtained antibacterial agent integrates the advantages of broad antibacterial spectrum, no drug resistance, hydrophily, environment friendliness and the like.