The invention provides an immunoassay method and a kit for detecting concentration of cyclosporine A. According to the immunoassay method, cyclosporine C is used as a competing molecule to quantitatively detect the concentration of cyclosporine A in a sample. The invention also provides an immunoassay kit for detecting concentration of cyclosporine A. By utilizing the immunoassay method provided by the invention to detect cyclosporine A, an anti-cyclosporine-A antibody with relatively low affinity can be used to obviously improve the analytical sensitivity, so that the detection result is high in accuracy and good in exactness.