Ultrasmall　nanoparticle　contrast　agents　provide　dual-mode　MRI.　However,　the　application　of　ultrasmall　nanoparticle　contrast　agents　is　limited　by　low　manufacturing　outputs　and　cumbersome　preparation　processes.　Herein,　we　report　a　novel　continuous-flow　coprecipitation　method　for　the　preparation　of　the　Fe3O4　nanoparticles　magnetic　fluid　(CFCPFe)　coated　with　ultrasmall　cysteine-terminated　polymethacrylic　acid　(Cys-PMAA).　The　preparation　process　is　more　coherent,　simpler,　and　less　expensive.　Compared　with　magnetic　fluids　prepared　by　the　conventional　method　(Cys-PMAA@Fe3O4),　CFCPFe　has　smaller　particle　sizes　(3.27　±　0.93　nm).　Moreover,　CFCPFe　demonstrates　excellent　stability　for　>180　days　with　different　pH　values　(pH　=　2–12)　and　salt　concentrations　(up　to　2　mol/L).　In　addition,　HEK293T　cytotoxicity　tests,　hemolysis　tests,　and　H&E　tissue　sections　show　excellent　in　vitro　and　in　vivo　biocompatibility.　In　vitro　magnetic　resonance　imaging　(MRI)　at　1.5　T　shows　that　the　r2　value　(50.51　mM−1·s−1)　of　CFCPFe　is　slightly　lower　than　that　of　Combidex　(r2　=　65　mM−1·s−1)　and　that　the　r1　value　(9.54　mM−1·s−1)　is　2.7　times　higher　than　that　of　Gd-DTPA　(r1　=　3.5　mM−1·s−1).　Finally,　in　vivo　imaging　shows　that　CFCPFe　reaches　the　tumor　region　of　the　mouse　liver　cancer　model,　and　a　small　tumor　can　be　observed　in　dual-mode　imaging.　This　work　offers　an　effective　method　for　the　preparation　of　a　low-cost,　stable,　and　biocompatible　ultrasmall　contrast　agent　exhibiting　a　strong　magnetic-imaging　effect　for　dual-mode　imaging.　©　2022