The switch window of demultiplexer using an electroabsorption modulator (EAM) is simulated and analyzed theoretically combined with the channel crosstalk and intensity fluctuations (synchronization deviation and time jitter). Results show that the characteristic of switch window (extinction ratio and window width) can be used as demultiplexer of 4×10 Gb/s; under small DC reverse bias, the tradeoff between the channel crosstalk and the optical transmissivity of the target signal has to be considered carefully; under large DC reverse bias, the maximum RF amplitude can lead to minimum bit error rate (BER).