Flexible　sensors　have　been　tremendously　attractive　in　wearable　electronics,　soft　robots　and　human-machine　interaction　devices.　In　this　work,　conductive　silicon　rubber　(CSR)　filled　with　silver-coated-glass　fiber　(AGF)　and　carbon　fiber　(CF)　was　embedded　into　polydimethylsiloxane　(PDMS)　matrix　to　fabricate　a　flexible　strain　sensor　(FSS)　in　a　3D-printing　(3DP)　process.　It　was　found　that　the　addition　of　CF　and　plasticizer　synergistically　formed　a　support-structure　which　guaranteed　the　CSR　to　be　extruded　out　easily　in　the　3DP　process.　The　printed　FSS　demonstrated　its　highly　stable　properties　with　a　gauge　factor　(GF)　of　8-10　at　different　strain　rate,　the　coincident　resistance　recovery　process　in　cyclic　load　of　different　strain,　as　well　as　the　stable　durability　with　monotonous　peak　in　600　strain-release　cycles.　Furthermore,　it　was　able　to　distinguish　a　superimposed　load　at　frequencies　of　1/12　Hz　and　1/36　Hz　with　aids　of　Fast　Fourier　Transformation　(FFT)　analysis.　Migrating　evolution　of　AGFs　in　a　stretched　CSR　was　observed　to　further　explore　and　paraphrase　the　mechanism　of　resistance　response　and　the　high　GF　of　the　FSS.　The　printed　FSS　was　capable　to　monitor　the　movement　of　various　joints　and　further　distinguish　a　superimposed　human　motion,　showing　its　potentially　practical　application　in　soft　robots　and　wearable　electronics.　(c)　2020　The　Authors.　Published　by　Elsevier　Ltd.　This　is　an　open　access　article　under　the　CC　BY-NC-ND　license　(http://　creativecommons.org/licenses/by-nc-nd/4.0/).