To accurately characterize the effect of the cavity surface roughness on the ohmic loss in gyrotron oscillators, this article proposes a method by using the gradient conductivity (GC) assessment model, in which a derived effective conductivity parameter is used to characterize the surface roughness. The results show that it has higher accuracy than the conventional methods such as the H-B formula, especially in the terahertz frequency band. This model also provides a reasonable explanation for the low-efficiency problem in terahertz gyrotrons, and it is beneficial to promote the design and analysis of gyrotrons.