The asymmetric X junction was applied in the GaAs Mach Zehnder optical switch instead of the traditional Y branch, and a 1× 4 switch array was designed and fabricated. The basic mechanism of the asymmetric X-junction and, then, of the switch array was briefly narrated. The consideration for the designing and fabricating process were presented. The switch unit in this device operating at 1 15 μm has a switching voltage of 12 V and a crosstalk of less than -20 dB. The propagation loss in the waveguide of this device is about 7 dB/cm. The factors causing the crosstalk and the loss in this device were analyzed.