To produce a high-performance microwave frequency comb (MFC), a tunable MFC generating scheme using optical comb heterodyning is proposed based on a cascaded Mach-Zehnder modulator (MZM) optical loop system. Continuous light injected through the coupler into the cascade MZM optical loop for modulation can generate an optical frequency comb (OFC), which beats in the photoelectric detector to produce MFC. A theoretical model is created to simulate the system. The influences of the light source power, light source line width, radio frequency driving signal (RF) frequency, and other parameters on the final MFC performance are studied with the optical communication system design software Optisystem. Results show that the output MFC generated by properly adjusting the parameters of the light source, RF, and other devices has a 300-GHz bandwidth, with flatness of 0.12 dB and power reaching up to 22.49 dBm. The scheme is structurally simple and easy to implement, while additionally obtaining a comb-tunable MFC when the RF signal frequency is adjusted.