With the development of wireless communication technology and micro-cell technology, optical-borne microwave technology, specially optical-borne multi-carrier technology has become one of the most important trends for generating high-quality sources. Therefore, the efficient generation of high-quality microwave signals has always been a requirement in wireless communication systems. Due to its low-noise and high-frequency output characteristics, photoelectric oscillator is widely used to generate high-quality microwave frequency sources in communication systems. Combining the advantages of photoelectric oscillator's low-noise output and direct-modulated laser's gain-switching state characteristics, a tunable optical-borne microwave frequency comb scheme based on dual-loop mixing-frequency photoelectric oscillator is proposed in this paper. And a direct-modulated laser operating in a gain-switching state is used to generate the original optical-borne microwave frequency comb signals. The dual-loop adjacent resonant frequencies are separated by two different high-frequency microwave bandpass filters. The beat frequency of adjacent frequencies mentioned above is injected back into laser to form photoelectric resonance, and thus enhancing the generated original optical-borne microwave frequency comb signals. To suppress the side modes caused by long resonant cavity, a polarized dual-loop structure is used in the system, and thus improving the noise characteristics of output signals. After experimental analysis, the dual-loop filtered resonant microwave signals and low-phase-noise microwave comb signals with a frequency interval of 797.4 MHz are all obtained. The microwave output side-mode suppression ratio after polarized dual-loop adjustment is improved to 47 dB. And microwave comb signal's first-order carrier phase noise is lower than-101.7 dBc/Hz at 10 kHz,-115.2 dBc/Hz at 50 kHz. In addition, higher-order carriers all come from the light multiplication of first-order carrier, they share the same low noise characteristics with first-order microwave comb signal. The output power of first-to-fourth, fifth-to-thirteenth order carriers are balanced to 10 dB by photoelectric resonance injection. And their side-mode suppression ratios are all better than 40 dB. Furthermore, theoretically, the comb interval can be adjusted to any frequencies by changing the center frequencies of two high-frequency bandpass microwave filters. Therefore, optical-borne multi-carrier microwave signals are generated efficiently and cost-effectively by this tunable optical-borne microwave frequency comb scheme, and the generated low-noise multi-carrier frequency sources meet the demand of an optical-borne microwave wireless communication system.