Electrochemical sensors have been highly developed and play an important role in the early diagnosis of different diseases. In this work, a simple and rapid electrochemical method was developed for the quantitative detection of a trinucleotide repeat (TNR) sequence d(CAG)_n based on magnetic beads and screen-printed carbon electrodes. Their combination greatly simplified the operation process. The intensity of the electrochemical signal was linearly correlated against the concentration of the TNR sequence d(CAG)_n in the range from 100 pM to 1 μM, with a limit of detection as low as 24 pM. Furthermore, the number of repeats of the TNR sequence d(CAG)_n could be detected and also showed a good linear relationship. The proposed method provides a promising rapid and simple method for the detection of a CAG TNR sequence for point-of-care testing in clinical applications.