To investigate the fire-extinguishing efficiency of liquid nitrogen in open space, this paper constructs a numerical model of a liquid nitrogen jet fire-extinguishing device through the AMESim multidisciplinary simulation platform, implementing research on the mass flow rate, the temperature of the heat leakage section, and the stabilization time of gas–liquid ratio concerning liquid nitrogen under diverse input pressures. Concurrently, this paper analyzes the deviation between the experimental results and the simulation results through the constructed experimental device of the liquid nitrogen jet system in the open space, thus exploring the impact exerted by different input pressures on the formation of the stable liquid nitrogen jet state of the system. Research findings indicate that increasing the input pressure of the system can effectively improve the stability of the liquid nitrogen jet. Specifically, in the case that the input pressure of the system exceeds 0.6 MPa, liquid nitrogen can be stably output, with a higher input pressure leading to a shorter time to produce liquid nitrogen at the nozzle. In the case that the input pressure of the system is 0.6 MPa, the liquid nitrogen jet demonstrates obvious intermittent fluctuation, so that it is impossible to form a stable jet.