We have observed that the apparent negative differential resistance(NDR) comes out in I-V characteristic curves of metal-insulator-metal(MIM) thin film tunnel junction when light-emission from this structure occurs in the experiments.The mechanism of this NDR is that the surface plasmon polariton(SPP),which plays a role of intermediate in the process of light-emission,creates the impeding effect upon tunneling electrons.We have also built the circuit simulation of electron-transfer, computed the numerical solution of I-V characteristic curve,and revealed the key action of SPP on the NDR in I-V characteristic curve and the light-emission from MIM tunnel junction.