The continuously spilling n-butanol fire is systematically investigated in steady flow state. The influence of thermal conductivity of substrate on spilling fire is revealed. According to the temperature distribution and spreading characteristics, the spilling fire over concurrent steady flow n-butanol is separated into the heat transfer dominant stage in low-discharge rate and the forced flow dominant stage in high-discharge rate with the critical discharge rate Q = 141.5 ml/min. The low-discharge rate spilling fire spread depends on the coupled effects of subsurface flow preheating and intrinsic dragging of flowing fuel, whereas the high-discharge rate spilling fire depends mainly on the dragging effect of non-slip boundary condition on the liquid surface. The temperature distribution of liquid surface proves that the behavior of thermal fluid diffusion occurs in high-discharge rate. The velocity of spilling fire decreases with an increase in thermal conductivity of substrate. The magnitudes of heat transfer involving spilling fire spread over n-butanol inside the subsurface flow namely gas conduction, liquid conduction, liquid convection, sensible heat and heat dissipation from substrate and walls of spilling trench are quantitatively calculated. The sum of sensible heat and convective heat loss accounts for more than 80% of total heat flux.