The invention belongs to the technical field of premixed polyether, and particularly relates to premixed polyether, an anti-ageing full-water type foam board and a preparation method of the anti-ageing full-water type foam board. The premixed polyether is prepared from the following raw materials: polyether polyol A, polyether polyol B, polyether polyol C, polyether polyol D, a cross-linking agent, an antioxidant, a catalyst, a foam stabilizer, a chemical foaming agent, a flame retardant and a foaming additive. The full-water type foam board prepared from the premixed polyether, which is disclosed by the invention, is also processed at normal temperature and under normal pressure, is simple in process, is easy to operate, is low in volatilization, does not generate three wastes (waste gas,waste water and waste solid), and is short in production period, low in cost, high in efficiency and low in energy consumption.