An ultrasensitive electrochemical detection method was developed to detect circulating tumor cells (CTCs) by using reduced graphene oxide/gold nanoparticles composites (rGO/AuNPs composites) as a support material with CuO nanozyme as a catalyst. MCF-7 circulating tumor cells were detected by an electrochemical cytosensor with effective surface recognition between specific mucin 1 protein (MUC-1) over-expressed on the MCF-7 cell membranes and MUC-1 aptamer. The CuO nanozyme is used as a signal-amplifying nanoprobe in an ultra sensitive electrochemical cytosensor to detect CTCs for the first time. Under the optimized experimental conditions, the proposed cytosensor exhibited significant analytical performance for the determination of MCF-7 circulating tumor cells. A wide detection range from 50 to 7 x 10(3) cells mL(-1) with a detection limit as low as 27 cells mL(-1) was reached on the condition of acceptable selectivity and reproducibility. Furthermore, the cytosensor can easily distinguish CTCs from the real serum sample due to the specific combination of MUC-1 and MUC-1 aptamer.