To reveal the influence of the formation and development of gas-phase streamer channel on liquid discharge between pin-plane electrodes, a numerical model of the transformer-oil discharge in the pin-plane electrode system is built based on the continuity equations of free charge carriers, which are coupled with the Poisson’s equation. The gas-phase processes during the streamer development process is also taken into consideration, including impact ionization and the increase in the mobility of free charge carriers in the gas-phase relative to the liquid-phase in the streamer channel. The Heaviside function is used to switch the simulation model between gas-phase and liquid-phase. The initial and propagation progress of streamer discharge under nano-second pulse voltage is simulated using the model. Simulating results show that the electric field at the streamer body is significantly reduced and the electric field at the head of the streamer is further enhanced with the addition of such low density gas-phase region. The propagation speed of the streamer in two-phase model is also faster than that of the ordinary liquid-phase model.