It discloses a polyether polyols for preparing flexible polyurethane foam, and a preparation method and application thereof. The method comprises the following steps: (1) carrying out a reaction on phosphorus oxychloride, epichlorohydrin, a first acidic catalyst and an inert solvent in a first microchannel reactor to obtain a chloroalkoxy phosphorus compound; (2) carrying out a reaction on the chloroalkoxy phosphorus compound, glycidol, a second acidic catalyst and an inert solvent in a second microchannel reactor to obtain a hydroxy compound; (3) carrying out a ring-opening reaction on the hydroxy compound, epoxy vegetable oil, a basic catalyst and an inert solvent in a third microchannel reactor to obtain a vegetable oil polyol; and (4) carrying out an addition polymerization reaction on the vegetable oil polyol, propylene oxide and an inert solvent in a fourth microchannel reactor to obtain the polyether polyols for preparing flexible polyurethane foam.