Sensing sensitivity is one of the crucial parameters for quartz crystal microbalance (QCM) sensors. Herein, we study the overtone mass sensitivity of a QCM sensor with an asymmetric N-M type electrode configuration. In order to overcome the deficiency that the sensitivity of the QCM sensor with an asymmetric electrode cannot be calculated by Sauerbrey's equation, we design the electrochemical electrodeposition experiments to measure it. The measurement results of overtone mass sensitivities of three 3.1-5.1 and three 4.1-5.1 QCMs are 5.418, 5.629, and 5.572 Hz/ng and 4.155, 4.456, and 3.982 Hz/ng in the third overtone mode and 9.208, 9.474, and 9.243 Hz/ng and 6.811, 7.604, and 6.588 Hz/ng in the fifth overtone mode, respectively. The overtone mass sensitivities of three 5.1-5.1 QCMs are 3.210, 3.439, and 3.540 Hz/ng in the third overtone mode and 5.396, 5.010, and 5.707 Hz/ng in the fifth overtone mode, respectively. These results show that the overtone mass sensitivity of the N-M type QCM is larger than that of QCMs with symmetric electrodes, and the fifth overtone mass sensitivity is higher than the third overtone mass sensitivity for the same type of QCM. The above results strongly confirm that the overtone mass sensitivity of a QCM sensor with an asymmetric N-M electrode structure significantly enhances its sensing performance, and it will greatly meet the demands for high precision measurement of QCM sensors in applications.