The present invention discloses a water-based environmental protection intumescent fire-retardant coating and its preparation method, which belongs to the technical field of functional coatings. Its characteristic is using water-based resin emulsion as the film-forming base material, adding composite dehydration foaming agent and charring agent with fireproof flame retardant effect according to the formula ratio, supplemented by flame retardant smoke inhibitors, reinforcing fillers, water and other additives. Fully grind and stir each material to make it mix evenly. The effect and benefit of the present invention is that the coating can form a uniformly foamed layer with good mechanical properties under high temperature or flame action, having good thermal insulation property, long fire resistance time, smooth coating surface, excellent water resistance and weather resistance, and simple preparation process flow.