Characteristics　and　formation　mechanism　of　filamentary　plasma　string　induced　by　single　picosecond　laser　pulse　in　sapphire　are　studied　experimentally　and　numerically.　Relative　brightness　and　spatial　distribution　of　the　filamentary　plasma　string　are　characterized　by　time-resolved　luminescence　images.　The　whole　filamentary　plasma　string　is　composed　of　a　leading　plasma　string　with　stronger　brightness　and　a　tailing　plasma　string　with　weaker　brightness.　The　numerical　analysis　shows　that　the　different　characteristics　of　filamentary　plasma　string　are　related　to　the　two　types　of　spatiotemporal　evolution　stages.　The　pivotal　role　of　avalanche　ionization　for　different　spatiotemporal　evolution　stages　is　revealed.　The　filamentary　plasma　string　induced　by　single　pulse　has　a　guiding　significance　for　the　subsequent　pulse　nonlinear　propagation　and　the　material　modification;　all　the　above　provides　basic　information　for　the　multi-pulse　filamentation　and　the　laser-induced　filamentation　processing　of　sapphire.