We have proposed a simple electrochemical method in this work for the assay of tumor cells through their own steric hindrance effect. Specifically, tumor cells can block the catalysis of terminal deoxynucleotidyl transferase to the aptamer previously immobilized on the electrode surface. By making use of the hindrance effect, cancer cells can be quantitatively analyzed in the range from 1.6 × 102 to 1.6 × 106 cells per mL without complicated design or cumbersome operation, while the detection limit can be about 53 cells per mL. This method can also show satisfactory performance in complex environments, indicating its potential in clinical application.