Flame spread over liquid fuel is an evitable process during the development of spilling liquid fires. The scale of the combustion zone may be expanded by the spreading fire, making the fire uncontrollable. Thus, the flame spread is a major threat to the process safety of the usage of liquid fuels. An experimental apparatus is designed to study the flame spread over n-butanol under the effect of the gas step that is built by the low-heat conductivity marbles. The flame morphology, flame resident time, and movement of subsurface liquid convection are measured under various widths and heights of the marble steps. The resident time is affected by the coupling effects of the viscous resistance of step and the heat loss to unburned fuel. The flame height is decreased by the width of the step but enhanced by the height of the step. The convective heat transfer through subsurface flow and flame radiation to the unburned fuel behind the step is calculated. The convective heat transfer rate accounts for more than 90% of the total heat transfer rate. This current finding provides a theoretical basis for the safety application and emergency management of liquid fuels.