Abstract: Choosing appropriate absorbing boundary holds the balance in analyzing the effects of optical elements on travelling light-wave based on Finite-Difference Time-Domain Method.Beginning from the Maxwell equations,analytical solution of one-dimensional absorbing boundary condition and its reflection coefficients are deduced in this article,perfect absorbing effect of first order Mur absorbing boundary and travelling-wave time-delay method are proved through computational simulation on the condition that the excitation source at the right truncation boundary or at the middle of discussed region exists as long as the time of TEM wave's propagation,and computation errors are also analyzed.It is proved that light-wave is stabilized early and has higher numerical precision under the Neumann boundary condition than Mur1 boundary condition.Both the Neumann and Mur1 conditions have excellent absorption character to TEM light-wave.