A　method　of　detecting　the　fundamental　positive　and　negative　sequence　components　is　proposed.　Based　on　the　instantaneous　symmetrical　components　and　coordination　transformation,　the　principle　of　detecting　the　fundamental　positive　and　negative　sequence　components　by　the　90°　phase　shift　operator　in　the　αβ　reference　frame　is　analyzed　theoretically.　A　phase　shift　system　based　on　SOGI(Second-Order　Generalized　Integrator)　is　developed.　Theoretical　analysis　and　simulative　results　show　that,　the　developed　system　has　the　characteristics　of　90°　phase　shift　at　special　frequency　and　serves　as　a　filter.　It　is　discretized　by　the　trapezoidal　method　and　used　to　detect　the　fundamental　positive　and　negative　sequence　components,　without　PLL　and　LPF.　Simulation　results　with　Matlab/Simulink　verify　the　correctness　and　effectiveness　of　the　proposed　method.