The　development　of　Surface　Enhanced　Raman　Scattering　(SERS)　promotes　the　wide　application　of　Raman　spectroscopy　in　chemical　and　biomolecular　detection.　SERS　detection　relies　on　analytes　in　close　contact　with　the　metallic　surface,　and　therefore　direct　molecular　trace　detection　in　the　liquid　phase　is　difficult.　In　this　paper,　static　liquid　phase　SERS　detection　was　performed　simply　using　a　capillary　without　pre-functionalization.　Gold　nanoparticles　(AuNPs)　with　an　optimized　size　ensure　localized　surface　plasmons　in　resonance　with　the　exciting　laser　light.　Grazing　incidence　and　multimode　interference　in　the　capillary　ensure　that　the　longitudinal　Raman　signal　is　effectively　excited　and　accumulated.　An　enhancement　factor　as　high　as　108　and　a　detection　limit　of　10-9　M　of　crystal　violet　in　aqueous　solution　have　been　achieved.　©　2021　The　Royal　Society　of　Chemistry.