The invention relates to combined polyether for flame-retardant bulk rigid polyurethane foams and an application method of the combined polyether and belongs to the technical field of the combined polyether. The combined polyether for the flame-retardant bulk rigid polyurethane foams is prepared from the following components including flame-retardant polyester polyol A, flame-retardant polyester polyol B, flame-retardant polyester polyol C, a crosslinking agent, a foam stabilizer A, a foam stabilizer B, a chemical foaming agent, a catalyst, a physical foaming agent, a flame retardant A and a flame retardant B. The combined polyether for the flame-retardant bulk rigid polyurethane foams is wide in raw material source and is low in cost. With the adoption of the combined polyether, the flame-retardant bulk rigid polyurethane foams prepared by the combined polyether are good in flame retardancy, low in thermal conductivity, good in airtightness and stable in quality performance. The invention also provides the application method of the combined polyether for the flame-retardant bulk rigid polyurethane foams. The application method is simple in process, is easily operated, is low in volatilization and is high in production efficiency without three wastes.