In an effort to improve the quantitative detection of aCL IgM, we develop a new immunoassay to improve aCL IgM detection based on TRFIA using the complex of cardiolipin plus bovine β2GPI as antigen and Sm(3+)-labeled rabbit anti-human IgM as conjugate. The precision, sensitivity, specificity, coefficient of recovery, and stability of the assay were evaluated and comparison with the classical ELISA was also made. The aCL IgM TRFIA kit we established had a wider detectable range than commercial ELISA ones when diluted a specimen with strong positive from 1:2.5-1:40960. We observed that for the established TRFIA kit there was a good liner range within 1:2.5-1:40960, whereas it was within 1:20-1:1280 when using ELISA kits. The intraassay precision rate and the interassay precision rate were <5% for 3 different concentrations. The sensitivity was 0.1MPL U/mL and the clinical diagnostic specificity was 98%. Average recovery rate was 101.13%. The established assay kit also behaved better in stability. Additionally, the immunoassay we established correlated well with the ELISA and the correlation coefficient was 0.956. We thus conclude that the TRFIA we developed for aCL IgM detection gives promise to a more sensitivity and reliable diagnosis of APS and has potential value for large-scale screening programs.