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 (8-4), and a spray pipe; the spray pipe comprises a contraction section (8-1), an expansion section (8-2), and an acceleration section (8-3) 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 (8-1) decreases, and an inner diameter of the expansion section (8-2) increases; an inner diameter of the acceleration section (8-3) is constant and equal to an outlet diameter of the expansion section (8-2); and the liquid nitrogen nozzle (8-4) communicates with the first input end and disposed on an axial line of the contraction section (8-1); an outlet of the liquid nitrogen nozzle (8-4) is coaxial with an outlet of the contraction section (8-1); 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 (8-1); 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 (8-4), and a spray pipe; the spray pipe comprises a contraction section (8-1), an expansion section (8-2), and an acceleration section (8-3) 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 (8-1) decreases, and an inner diameter of the expansion section (8-2) increases; an inner diameter of the acceleration section (8-3) is constant and equal to an outlet diameter of the expansion section (8-2); and the liquid nitrogen nozzle (8-4) communicates with the first input end and disposed on an axial line of the contraction section (8-1); an outlet of the liquid nitrogen nozzle (8-4) is coaxial with an outlet of the contraction section (8-1); 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 (8-1); 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 . <EOS>