In order to understand the ignition process of plasma jet in the liquid propellant electrothermal chemical gun， the spreading characteristics of plasma jet in simulative liquid propellant LP1846 were studied. A two-dimensional axisymmetric unsteady mathematical model of plasma jet spreading in the liquid was established， and the model was validateded with the experiments based on the liquid working medium of water. On this basis， the spreading process of plasma jet in the simulative liquid propellant LP1846 was numerically simulated. The morphological changes of plasma jet and the distribution characteristics of pressure， velocity and temperature in the jet field were analyzed. Results show that when the plasma jet expands in the simulative liquid propellant LP1846， there is turbulent mixing phenomenon due to Taylor-Helmholtz instability， and it becomes more and more intense. It is manifested by the protruding head of the jet and the axial elongation to form a tip， and the jet entrains the simulative liquid propellant medium to produce droplets in Taylor cavity， and the number of droplets gradually increases. At the same time， the necking phenomenon occurs near the nozzle hole during the expansion of plasma jet due to the simulative liquid propellant backflow. The jet field fluctuates due to the alternating action of expansion and compression waves， especially near the nozzle hole. The pressure field shows alternating distribution of high and low pressures， and the velocity field also shows similar distribution.