The experimental principle of Functional Simulation （FS） was proposed aiming at the fire in the space shuttle caused by the electrical wire overload or short circuit because it is of great important significance of study on the wire insulation flammability in microgravity for fire safety. The concept and conditions of the FS were presented in this paper, Experimental setup was established according to the FS concept. The study on the fire precursor of the wire in microgravity was accomplished by the proposed experiment facility at normal gravity. The experimental results show that the equilibrium temperatures of wire insulation were much higher in the microgravity than that in the normal gravity condition with the overload currents because the convection is greatly reduced and suppressed, Meanwhile, the wire was molten down and the insulation was burnt observed in the experiments. These results furtherly confirmed that the wire is the potential ignition source in the case of overloading or short circuit in the space shuttles.