The invention discloses a method for preparing nano silver powder in viscous medium. The method comprises the following steps of: mixing sodium nonylphenol polyoxethyiene ether sulfate, TEA dodecylbenzenesulfonate, ammonium dodecylbenzenesulphonate, coconut oil alcohol acylamide, octadecyl dimethyl amine oxide, sodium carboxymethylcellulose, polyoxyethylene glycol 6000 bisstearate, ethylenediamine tetraacetic acid disodium salt, ethylene oxide, benzotriazole and deion water and stirring uniformly, preparing the viscous medium, adding silver acetate and silver lactate and stirring uniformly, adding sodium hypophosphite and stirring till full reaction, then adding the deion water for dilution according to a ratio of 1:1.5 of the all material amount to the deion water, reducing the viscosity, and obtaining the nano silver powder by pressure filtration, washing with the deionized water, washing with acetone and vacuum drying. The selected raw materials are easily obtained; the method is short in production process, high in production efficiency, low in production cost and low in energy consumption; the powder is not easily agglomerated, and the powder has good dispersibility; and the method is suitable for large-scale production, and solves the problems that the nano silver powder prepared by the conventional chemical preparation method is easily oxidized and easily agglomerated.