The high-temperature objects widely exist in industrial and civil building such as the nearby burning pool fires, the heated walls and the hot smoke layer accumulated on ceilings. The output of radiant energy from the adjacent high-temperature objects may accelerate the flame spread. In this paper, a set of well-designed laboratory-scale experiments are carried out to reveal the combustion behavior and flame spread over jet fuel of RP-3 with different radiant heat fluxes and radiant exposure times. Several characteristic parameters, namely flame height, flame spread rate, fuel surface temperature and velocity of subsurface flow, are quantified and analyzed to describe the flame spread behaviors. The average flame height is augmented by the time-integrated radiant heat flux due to the accumulative heat flux and the expanded combustion area. Similarly, measurements of flame spread rate and liquid fuel temperature identify that they are sensitive to the applied radiant flux, but this sensitivity decreases as the flame volume and the blockage effect increase. The external radiant heat flux can simultaneously influence the flame spread from both gas and liquid phases. The thermal exchange and fluid motion involving flame spread under varied external radiant conditions are revealed. The theoretical formula between the fuel surface temperature and the time-integrated radiant heat flux is established based upon the energy conservation law and the thermal boundary layer theory. The current results possess the practical guiding importance for the development of liquid spilling fire disposal measures under the radiations of external heat flux.