Abstract: In this paper, the field analysis method for calculating the characteristic parameters of the output circuit in the klystron is presented. The electromagnetic component under proper assumptions and the simple equivalence is studied by three-dimension electromagnetic simulation software ISFEL3D. The theoretical analysis and the calculation show that the field analysis method can provide the calculation of characteristic parameters such as gap-impedance, coupling degree, efficiency of output cavity, resonance frequency, characteristic impedance and so on in the output circuit of the klystron. The coincidence between the cold test data and the calculation result to a certain extent preliminarily verified the reliability of the field analysis method.