For simulating the ignition transient in solid rocket motor (SRM) for high energy propellant,process of flame-spreading and the initial spreading speed of the gaseous phase or the solid phase are necessary. Two flame-spreading experiments for back-forward facing step grain were carried out where a testing method based on thermocouple target-line and optical measurement methods were used. Results show that upward region of the propellant surface is ignited by the flame-spreading process,backward region is ignited by the gas of the upward propellant burning,and the step bottom region is ignited at last. The time of ignition transient by the resistance wire is so long that the speed for the upward gas to the backward region when burning is 57.78 cm·s^-1,while the speed of flame-spreading on the upward region is only 4.92 cm·s^-1.