We　report　the　direct　laser　writing　(DLW)　of　surface-enhanced　Raman　scattering　(SERS)　structures　on　the　inner　wall　of　a　hollow　fiber.　Colloidal　gold-silver　alloy　nanoparticles　(Au-Ag　ANPs)　are　firstly　coated　onto　the　inner　wall　of　a　hollow　fiber.　A　green　laser　beam　is　focused　through　the　outer　surface　of　the　hollow　fiber　to　interact　with　colloidal　Au-Ag　ANPs　so　that　they　become　melted　and　aggregated　on　the　surface　of　the　inner　wall　with　strong　adhesion.　Such　randomly　distributed　plasmonic　nanostructures　with　high　density　and　small　gaps　favor　the　SERS　detection　of　low-concentration　molecules　in　liquids　flowing　through　the　hollow　fiber.　Such　a　SERS　device　also　supplies　a　three-dimensional　microcavity　for　the　interaction　between　excitation　laser　and　the　target　molecules.　The　DLW　system　consists　mainly　of　the　flexible　connection　between　the　motor　shaft　and　the　hollow　fiber,　the　program-controlled　translation　of　the　hollow　fiber　along　its　symmetric　axis　and　rotation　about　the　axis,　as　well　as　the　mechanical　design　and　the　computer　control　system.　This　DLW　technique　enables　high　production,　high　stability,　high　reproducibility,　high　precision,　and　a　high-flexibility　fabrication　of　the　hollow　fiber　SERS　device.　The　resultant　microcavity　SERS　scheme　enables　the　high-sensitivity　detection　of　R6G　molecules　in　ethanol　with　a　concentration　of　10(-7)　mol/L.