The switching characteristic of a semiconductor optical amplifier based demultiplexer with symmetric Mach-Zehnder Interferometer (SOA-MZI) operating at colliding pulse Mach-Zehnder (CPMZ) and travelling pulse Mach-Zehnder (TPMZ) configuration is analyzed. It shows that the control pulse width, the SOA length effect and the nonlinear gain compression effect influence the width of the switching window, and they are the main factors in limiting the switching speed of the CPMZ configuration. When the control pulse width is less than the SOA transition time, switching contrast ratio of CPMZ will begin to decrease if the delay between the two control pulses is less than the twice of SOA transition time. However when the control pulse width exceeds the SOA transition time, the switching contrast ratio will decrease even the delay is larger than the twice of SOA transition time. Hence a short control pulse and a delay larger than the twice of SOA transition time are required to enable the CPMZ to operate at a high speed with a high switching contrast ratio. While TPMZ configuration is almost free from the SOA′s length effect.