The　cost　and　the　efficiency　of　processing　data　for　a　structural　health　monitoring　system　could　both　be　improved　by　placing　a　proper　number　of　sensors　on　the　import　locations　of　a　observed　structure.　The　minimum　number　of　sensors　for　observing　a　normal　structure　and　a　structure　with　repeated　frequencies　is　studied　respectively　based　on　dynamic　system　observability.　A　method　to　solve　maximum　Fisher　information　matrix　with　Frobenius　norm　is　presented.　The　conclusion　that　solving　maximum　Fisher　information　matrix　is　equal　to　selecting　the　subset　of　modal　vectors　with　maximum　energy　is　drawn.　Considering　multiple　criterions　for　modal　analysis,　an　optimal　placement　of　sensors　method　based　on　nodes　energy　and　modal　assurance　criterion　is　presented.　The　results　given　by　numerical　example　show　that　the　method　proposed　is　valid　and　effective.