To study the horizontal fire spread characteristics of transformer oil, a series of experiments were carried out on the experimental platform developed. The influence of the initial temperature and the width of the oil pool on the flame propagation, including the propagation speed, flame morphology, and the temperature field distribution of the gas–liquid two phase, was analyzed to reveal the flame propagation characteristics and the oil surface temperature rise law in the process of transformer oil fire propagation. A theoretical model of coupled liquid-phase convective heat transfer and flame radiation heat transfer was established by combining thermodynamic theory to quantitatively calculate the heat transfer process of surface flow in the flame propagation process. The results show that the main flame dominated by diffusion combustion and flash flame dominated by premixed combustion are formed during the spread of transformer oil fire, and the flame spreads forward in a pulsating form. There is a step heating phenomenon during flame spreading forward. The initial temperature and oil pool width affect the flame propagation speed and flame pulsation frequency via changing the heat transfer on the high-temperature surface. Surface flow is mainly driven by surface tension, and the proportion of liquid-phase convective heat transfer to the total heat flow is much larger than flame radiation. Liquid-phase convective heat transfer is the main mode of surface flow heat transfer.