When electric field is measured in the bounded-wave EMP simulator experiment, one kind of optical fiber measurement system is often used. At the same time, the effect of optical transmitter on electromagnetic wave distribution is often concerned. In this paper, we assume that the optical transmitter is placed on the working region of the bounded-wave EMP simulator and the Finite-Difference Time-Domain (FDTD) method is used to model the effect of optical transmitter on electromagnetic wave distribution. From the numerical result, we can find the law of effect of optical transmitter on electromagnetic wave distribution.