The invention discloses a method and device for determining a well mouth injection phase state of fluid. The method comprises the step of acquiring a critical condition of a well bottom stratum of an injection well, wherein the critical condition at least comprises a safety limiting condition at the well bottom stratum, a flow limiting condition and design temperature of the well bottom, and the injection well is used for injecting the fluid into various injection strata; the obtained critical condition is converted to a boundary condition of the well mouth of the injection well; and according to the conversion result of the boundary condition of the well mouth, the injection phase state of the well mouth of the fluid is determined. According to the method and the device disclosed by the invention, the technical problem that the well mouth phase state design in related technologies is unreasonable is solved.