The mathematic model of strut was established on the model of Yee grid in three-dimensional space and on the model of leapfrog in time-domain. The formula of strut current and inductance per unit length in three-dimensional Cartesian coordinates were deduced. The realization of struts in different orientations and different coordinate systems in two-dimensional and three-dimensional space was discussed. At the same time the iterative process under main finite-differential time-domain iteration of strut was given. Finally， magnetically insulated transmission line oscillator was used as an example to validate the simulation for output power and particle movement.