A sensitive immunoassay based on immunomagnetic beads for measuring carcinoembryonic antigen (CEA) concentration in cancer cells was presented in this article. We used immunomagnetic beads to separate CEA in the gastric cancer cells and those CEA could be marked with horseradish peroxidase (HRP) labeled anti-CEA secondary antibody. The HRP could catalyze non-fluorescent 10-acetyl-3,7-dihydroxyphenoxazine (ADHP) with hydrogen peroxide (H2O2) to fluorescent products. Therefore, the CEA concenctration was quanlified by flourescence intensity at 585 nm emission wavelength. The flourescence intensity of the product responged well linearly to CEA concentration in the range from 3.0 to 60 ng/mL with a lower detection limit of 1.5 ng/mL(3σ). This method could be applied to determine tumor markers in gastric cancer cells.