Abstract: The theoretical analysis to the RF cavity at China Spallation Neutron Source (CSNS), a typical ferrite-loaded coaxial resonant cavity operating in swept condition by using the coaxial transmission line theory is presented. The modeling method of the cavity in the CST three-dimensional simulation software is also described in details, especially about the use of ceramic insulators representing resonant capacitors. The characteristics of the cavity obtained by the coaxial transmission line method, the CST simulations and the measurements have been compared, and are consistent. The parasitic mode induced by the Busbar has been studied by the CST simulations and the measurements, and different improvement methods over the prototype cavity are given. The impact of the filling factor of ferrite rings on the frequency tuning in the cavity has also been analyzed, and it turns out that a larger tuning range for the bias current is needed for a shorter cavity.