A multifunctional fire engine with liquid nitrogen as a jet power, comprising a vehicle frame, a liquid nitrogen storage tank, a liquid nitrogen conveying pipeline, a gasification device, a plurality of electric valves, a water pipe adapter, a liquid nitrogen spray gun, and a mixed spray gun; wherein: the liquid nitrogen storage tank is disposed on the vehicle frame; the liquid nitrogen conveying pipeline comprises at least a first pipeline and a second pipeline; the first pipeline connects a lower part of the liquid nitrogen storage tank, the gasification device, and an upper part of the liquid nitrogen storage tank sequentially in that order; the second pipeline connects the liquid nitrogen storage tank, an input end of the liquid nitrogen spray gun and a first input end of the mixed spray gun sequentially in that order; the mixed spray gun comprises a first input end, a second input end, a liquid nitrogen nozzle, and a spray pipe; the spray pipe comprises a contraction section, an expansion section, and an acceleration section which are connected to one another in that order; along a direction from the contraction section to the acceleration section, an inner diameter of the contraction section decreases, and an inner diameter of the expansion section increases; an inner diameter of the acceleration section is constant and equal to an outlet diameter of the expansion section; and the liquid nitrogen nozzle communicates with the first input end and disposed on an axial line of the contraction section; an outlet of the liquid nitrogen nozzle is coaxial with an outlet of the contraction section; an inlet of the second input end is connected to the water pipe adapter and an outlet of the second input end communicates with the contraction section; and the plurality of electric valves is disposed on the liquid nitrogen conveying pipeline and a water delivery pipeline connected to the water pipe adapter. the liquid nitrogen storage tank is disposed on the vehicle frame; the liquid nitrogen conveying pipeline comprises at least a first pipeline and a second pipeline; the first pipeline connects a lower part of the liquid nitrogen storage tank, the gasification device, and an upper part of the liquid nitrogen storage tank sequentially in that order; the second pipeline connects the liquid nitrogen storage tank, an input end of the liquid nitrogen spray gun and a first input end of the mixed spray gun sequentially in that order; the mixed spray gun comprises a first input end, a second input end, a liquid nitrogen nozzle, and a spray pipe; the spray pipe comprises a contraction section, an expansion section, and an acceleration section which are connected to one another in that order; along a direction from the contraction section to the acceleration section, an inner diameter of the contraction section decreases, and an inner diameter of the expansion section increases; an inner diameter of the acceleration section is constant and equal to an outlet diameter of the expansion section; and the liquid nitrogen nozzle communicates with the first input end and disposed on an axial line of the contraction section; an outlet of the liquid nitrogen nozzle is coaxial with an outlet of the contraction section; an inlet of the second input end is connected to the water pipe adapter and an outlet of the second input end communicates with the contraction section; and the plurality of electric valves is disposed on the liquid nitrogen conveying pipeline and a water delivery pipeline connected to the water pipe adapter.