The　imaging　character　of　microsphere　is　determined　by　its　refractive　index　and　size.　In　this　paper,　it　is　demonstrated　that　the　spherical　aberration(S)　and　numerical　aperture　(NA)　of　the　microsphere　are　different　with　different　refractive　index　or　different　radius.　The　relationship　between　the　focal　length　and　refractive　index　is　obtained　by　numerical　simulation.　It　is　shown　that　the　focal　length　decreases　with　the　increase　of　the　refractive　index.　The　focal　length　and　diameter　possesses　a　linear　relation,　which　is　matched　with　the　theoretical　result.　It　is　also　found　that　the　photonic　nano-jet　may　appear　when　the　refraction　index　and　size　of　the　microsphere　are　chosen　properly.　The　microsphere　has　great　potential　in　the　laser　processing　of　surfaces　materials　and　super-resolution　detection　of　nano　structure　etc.　So　the　study　on　the　microsphere　imaging　has　great　significance.