Discharge in transformer oil is one of the primary causes of transformer failures. The physical mechanisms of oil discharges are not yet fully understood. However, the prevailing electrohydrodynamics models primarily adopt field ionization and collision ionization, neglecting photoionization, which could potentially affect the discharge of liquid dielectric streamers. To reveal the discharge characteristics of transformer oil, this paper improves the field ionization term in electrohydrodynamics model, establishes a function of the ionization energy of transformer oil in relation to the recombination strength of carriers, and proposes a simulation model of streamer discharge in oil considering photoionization. The effects of photoionization on the characteristics of streamer discharge in transformer oil are studied, by examining the morphology and propagation velocity of streamers. The results show that the introduction of the photoionization mechanism promotes the formation of streamer branches and makes the energy in the streamer channel more concentrated. Moreover, photoionization facilitates the rapid propagation of streamers, which is key to the formation of the third mode of streamers.