To deeply understand the combustion and propulsion mechanism in liquid propellant electrothermal chemical gun, an experiment device was designed.The expansion process of injecting the pulsed plasma jet into the liquid medium was observed by a high-speed video system. The changes in axial displacement and volume of Taylor cavity formed by the interaction of plasma jet and liquid medium were measured. The quantitative expansion law for the change in Taylor cavity volume with time was obtained. The effects of liquid chamber boundary shape, discharge voltage and nozzle diameter on the pulsed plasma jet expansion were discussed. Results show that the non smooth intefacer with wrinkles in junction of two-phase due to the turbulent mixing is formed. There is an obvious oscillation phenomenon in the expansion process of Taylor cavity. The stepped-wall structure factor decreases from 0.6 to 0.4, the axial displacement first peak and the volume first peak increase by 16.6% and 12.4%, respectively. The discharge voltage decreases from 3000 V to 2100 V, the axial displacement first peak and the volume first peak reduce by 36.1% and 60.7%, respectively. The nozzle diameter decreases from 2 mm to 1.5 mm, the axial displacement first peak and the volume first peak reduce by 27.2% and 56.7%, respectively.