Abstract: Based on the barrier model of metal junctions, the radiation characteristics of metal junctions and the process of dynamic balance of the direct current bias voltage are analyzed in this paper. After the metal junction becomes stable, the expression of the third harmonic current component is given. Then from the perspective of metal junctions, the factors which influence the radiation characteristics of the third harmonic current component of metal junction, are discussed through a detailed numerical analysis. Based on the theoretical and simulation results, the fact is found that when the thickness of the insulating film grows larger, the contact area of the metal and the oxide layer goes smaller, or the barrier height between the metal and the oxide layer surface becomes larger, a smaller third harmonic can be obtained. The experimental results show that the smaller contact area of the metal and the oxide layer, as well as the larger barrier height between the metal and the oxide layer surface, leads to the smaller third harmonic.