Frequency　and　displacement　modal　indices　are　inefficient　to　reflect　structural　damage　when　using　conventional　finite　element　or　experiment　analysis　for　damage　diagnosis　of　truss　structure.　In　order　to　improve　efficiency　and　reliability　for　damage　diagnosis,　based　on　the　characteristics　of　space　truss　structure,　element　strain　modal　can　be　obtained　by　nodal　displacement　modals　by　means　of　the　finite　element　method　on　space　truss　element　connected　by　pin　between　two　nodes.　Therefore　the　change　of　elemental　strain　modal　is　presented　as　new　dynamic　indicator　for　damage　locations　of　space　truss　structure.　Furthermore,　damage　degree　can　be　derived　from　the　value　of　the　change　of　damage　elemental　strain　modal.　A　numerical　example　for　a　spatial　truss　is　provided　to　verify　the　practicability　of　the　method.　The　results　show　that　it　is　effective　to　locate　damage　and　assess　damage　degree　only　by　lower　modes.　Moreover,　the　results　indicate　that　the　proposed　method　based　on　strain　modal　of　truss　element　can　accurately　identify　damage　location　of　space　truss　structure　with　the　noisy　modal　data.