Based on the single-periodic and multi-periodic oscillation states of an optically injected semiconductor laser, a scheme for the generation of an ultra-broadband microwave frequency comb through the cascaded optical injection of semiconductor lasers is proposed. In this scheme, the optical frequency comb produced from a slave laser with continuous-wave injection is injected into another one, to generate a microwave frequency comb with much wider bandwidth due to the optical injection effect of laser. The numerical investigation based on the rate equations of an optically injected semiconductor laser shows that, as for the second optical injection and the selection of appropriate parameters, the bandwidths of this microwave frequency comb reach 52, 65 and 97 GHz within amplitude fluctuations of ±2.5 dB, ±5 dB and ±10 dB, respectively. Therefore, the proposed cascaded injection scheme is effective in generating ultra-broadband microwave frequency combs with more flat property.