The experimental study focuses on the behaviors of two-dimensional flame propagation over diesel and diesel-wetted sand beds. Effects of the diesel depth, ignition position, fuel ratio of diesel volume to sand bed weight and sand diameter on the flame spread are analyzed. The results show that for wetted sand beds, the flame spread rate increases with increasing fuel ratio and decreasing sand diameter. The capillary rise effect still plays a significant role with different fuel ratios. Considering the flame spread rate, flame height, temperature distributions near fuel surface and heat fluxes around the fire, the controlling mechanisms of heat transfer are discussed. For the low fuel ratio, the flame spread over wetted sand beds is dominated by the capillary rise effect and heat conduction of sand beds. With increasing fuel ratio, the main controlling mechanism changes from flame radiation to the combined liquid convection and flame radiation. The spread rate for diesel ignited from pan edge is greater than that ignited from pan center for fuel depths (10–20 mm). The flame spread rate increases due to the effect of flame radiation to the unburned fuel surface. For diesel cases ignited from pan center, the dominant mode of heat transfer to the unburned surface is by flame radiation for 2 mm depth, while the controlling mechanism is liquid convection for deep layers (5–20 mm). For diesel cases ignited from pan edge, flame radiation becomes the dominant heat transfer mode for the very thin and thick depths (2 mm and 20 mm), while the controlling mechanism is liquid convection for 5 mm depth. For deeper fuel layers, the flame radiation generates its effect primarily by raising the fuel surface temperature through extensive preheating region. The present experimental results can provide practical guide for the combustion hazard of accidental fuel spills.