The invention relates to a nano exterior wall coating; the composition by the weight percent is as follows: 40 percent to 50 percent of film-forming latex, 26 percent to 38 percent of pigment and filler, 4 percent to 8 percent of assistant, 0.3 percent to 1 percent of nano powdery material, 1.0 percent to 3.0 percent of water-soluble nano silica sol, and 10 percent to 30 percent of deionized water. The film-forming material can be acrylic latex or pure siloxane-acrylate latex; the pigment and filler is one or more than one type among titanium white powder, mica powder, barium sulfate and aluminum silicate; the assistant comprises one or more than one type among dispersant, wetting agent, foam killer, preservative, thickener, anti-freezing agent, ph regulator and film-forming assistant. Thecoating of the invention can be washed and brushed for 50000 times without breakage, resist manual climate aging for 1000 times without bubbling, peeling and crackling, has obvious mildew resistant effect and can effectively prevent mildew after being stored and coated on the wall.