In order to study the jet characteristics of liquid nitrogen fire nozzles, a mathematical model and a geometric model of a twophase nozzle with mixed jet of nitrogen and water were established. The numerical simulation of the velocity, pressure and density of the gasliquid twophase flow field was carried out by computational fluid dynamics. The nozzle inlet, outlet and total length of the simulation parameters were changed, the the jet speeds of different parameters were compared, and the optimum nozzle parameters were obtained. The results of the pressure simulation cloud chart indicate that the nozzle is prone to negative pressure at the bend, so the design of the nozzle should be designed to be curved.