The invention relates to an all-water soft flame-retardant combined polyether, a low density polyurethane continuous slab prepared from the all-water soft flame-retardant combined polyether, and a preparation method of the low density polyurethane continuous slab. The all-water soft flame-retardant combined polyether is prepared from a first polyether polyol, a second polyether polyol, a flame-retardant polymer polyol, a chain extender, a foam stabilizer, a catalyst, a chemical foaming agent, and a flame retardant. The beneficial effects are that: water is taken as the foaming agent, so the ozone layer is protected from being destroyed; the obtained low density polyurethane continuous slab possesses relatively low density, excellent flame resistance and size stability, and no shrinkage ordeformation is caused.