The interaction between nanomaterials and biological macromolecules (including protein) is the important basis of biological safety assessments of nanomaterials. In this paper, the fluorescence spectroscopy, synchronous fluorescence spectroscopy and circular dichroism spectroscopy) were used to analyze the interaction between four water-soluble carboxyl carbon nanotubes (long-SWCNTs-COOH, short-SWCNTs-COOH, DWCNTs-COOH and MWCNTs-COOH) and human serum albumin. Results showed that the four water-soluble carboxylated carbon nanotubes could quench the intrinsic fluorescence of human serum albumin at different extents. Under the same concentration, the quenching ability of four carboxylated carbon nanotubes were in the order of DWCNTs-COOH