Based on a simple infinite dimensional electromagnetic system consisting of a linear lossless transmission line in combination with a nonlinear boundary condition, the local map of backward voltage wave is established by using the traveling wave theory. The numerical simulation results show rich spatiotemporal nonlinear phenomena in the voltage wave on the lossless transmission line. The pattern dynamics in spatiotemporal chaos of the voltage wave on the lossless transmission line are qualitatively analyzed by depicting space-amplitude plot and space-time diagram. A good model with solution is established to study and understand the spatiotemporal chaos.