Abstract： In this paper, the physical characterestics of the interaction in a gyrotron with a longitudinally varying magnetic field is presented. Equations of motion used in reference are improved by expressing them in normalized form and making them more accurate and more generally applicable. Numerical results are obtained for a pratical case. The physical mechanism of efficiency enhancement with varying magnetic field and its effect on the gyrotron operation are discussed. Phase bunching diagrams demonstrate that the saturation phenomena will be delayed when such varying magnetic field is present.