On　chip　photonics　devices　relying　on　the　weak,　volatile　thermo-optic　or　electro-optic　effect　of　silicon　usually　suffer　from　high　energy　consumption　and　low　refractive　index　coefficient.　In　this　paper,　we　designed　a　1　2　phase　change　optical　switch.　The　device　is　implemented　in　two　silicon　waveguides　system　using　an　overcladding　layer　of　the　phase　change　data　storage　material　Ge2Sb2Te5　(GST),　which　exhibits　high　contrast　in　its　optical　properties　upon　transitions　between　its　crystalline　and　amorphous　structural　phases.　The　switch　shows　cross-state　and　bar-state　corresponding　to　amorphous　GST　(a-GST)　and　crystalline　GST　(c-GST),　respectively.　The　characteristic　parameters　of　the　switch　were　carefully　designed　and　simulated　by　three-dimensional　finite-difference　time-domain　(3-D　FDTD)　method.　As　the　simulation　result,　the　insert　loss　was　less　than　0.5　dB.　The　crosstalk　was　-16　dB　for　cross-state　and　-29　dB　for　bar-state　at　1550nm.The　device　performance　can　be　further　optimized　and　the　low　consumption　phase　change　optical　switch　can　be　expected　for　future　optical　communication　networks.　©　Published　under　licence　by　IOP　Publishing　Ltd.