A novel well-defined plasma assisted combustion system with in-situ discharge on the counterflow flame was developed to study the direct coupling kinetic effect of the in-situ non-equilibrium plasma on the flame. The uniform discharge was generated between the two burner nozzles. The ignition and extinction characteristics of CH4/O2/He diffusion flame were investigated by monitoring the OH∗ emission intensity at fixed strain rate and O2 concentration at the oxidizer side while varying the fuel mole fraction. A transition of ignition and extinction diagram from the conventional S-shape with both ignition and extinction limits to the monotonic and fully stretched S-shape without extinction limit was found at conditions with different O2 concentrations in the non-equilibrium plasma discharge. The monotonic and fully stretched "S" curve indicated clearly that the plasma generated reactive species caused a transition from extinction limited flame stabilization to ignition limited flame stabilization, that is, the extinction limit does not exist until the ignition limit is reached. The Rayleigh scattering measurement showed the local maximum temperature in the reaction zone at which ignition happened can be as low as 900 K.