With　the　development　of　Earth　and　Space　missions,　the　quest　for　higher　spatial　resolution　for　telescopes　will　turn　into　urgent　need.　Optical　sparse-aperture　system　can　be　designed　to　obtain　high　resolution　in　astronomical　object　imaging　with　several　independent　small-aperture　telescopes.　However,　the　images　from　separated　telescopes　are　not　only　overlapped　but　also　have　same　phase.　The　analysis　and　discussion　on　system　error　which　would　affect　the　co-phasing　is　presented.　The　piston　error,　namely　the　relative　longitudinal　phase　shift　across　the　sub-aperture　is　an　important　error　to　affect　the　performance　of　the　imaging　system.　In　this　paper,　we　demonstrate　the　effects　of　the　piston　error　on　the　point　spread　function　of　the　system.　Then　the　simulation　has　been　fulfilled　with　the　specific　system　parameters.　The　direct　imaging　results　are　obtained.　Due　to　the　image　restoration　techniques,　we　can　also　produce　higher　resolution　and　better　imaging　quality　result　after　the　restoration.　By　using　the　correlation　coefficient　as　the　evaluation　criterion　of　imaging,　the　results　show　that　image　quality　is　reducing　with　the　increase　of　the　piston　error　periodicity.　©　2009　SPIE.