Optical　sparse-aperture　imaging　system　is　one　of　the　most　potential　techniques　which　can　break　through　the　physical　limits　to　the　diameter　of　conventional　filled　aperture　telescope.　The　image　from　individual　telescope　not　only　should　be　overlapped,　but　also　have　the　same　phase　at　image　plane.　The　purpose　of　this　paper　is　to　study　the　cophasing　errors　which　would　degrade　the　performance　of　sparse-aperture　imaging　system.　The　analysis　and　discussion　on　system　errors　which　would　lead　to　the　cophasing　error　is　presented,　including　piston　error,　tip/tilt　error　and　pupil　mapping　error.　The　results　of　the　numerical　simulation　are　presented　on　the　system　which　has　two　sub-systems　and　certain　parameters.　The　effects　of　the　piston　error　and　tip/tilt　error　on　the　point　spread　function　(PSF)　of　the　system　are　discussed.　The　image　degradation　due　to　the　piston　and　tip/tilt　errors　are　shown　based　on　the　imaging　system　which　has　three　subsystems.