Disclosed are an oil-in-water emulsion defoamer and a method for preparing the defoamer. The defoamer may have a dispersed oil phase with a droplet size of 4 to 8 μm, and a viscosity of 100 to 1000 cP, and is widely used for defoaming in paper coating and wet-end in a paper mill. A method for preparing an oil-in-water emulsion defoamer is also provided. The method may include adding an emulsifying agent to biodiesel, and mixing, then gradually adding hydrophobic fumed silica to obtain a first mixture. The method may also include gradually adding water and a non-ionic surfactant, and stirring until uniform to obtain a second mixture. The method may also include gradually adding the first mixture to the second mixture, and stirring until uniform, to obtain an oil-in-water emulsion defoamer.