In this paper, we present a method to rapidly discretize a continuous time-variant system in real time. Three discretization methods are compared. The proposed method does not calculate for discretization, and therefore, the calculation cost is 1/100 of that of the conventional zero-order hold model. The presented method has high stability in higher order systems and the position and size of the internal parameters are determined for each sampling time. The effectiveness of the Finite-Difference Time-Domain (FDTD) method is demonstrated by comparing responses with a continuous system by using simulations. By experiments, we reproduce the haptic sensations based on the time-variant environmental model expressed by Tustin approximation and FDTD method.