In order to obtain microwave signal with high spectral purity, low phase noise and flexible tunability, a novel approach to achieving a tunable optoelectronic oscillator (OEO) which was based on optically injected semiconductor laser and subharmonic microwave modulation for microwave signal generation was proposed and experimentally demonstrated. The fundamental concepts for realizing the OEO were based on the wavelength-selective amplification effect and the period-one(P1) oscillation state of optically injected semiconductor laser. The frequency stability, side-mode-suppression ratio and spectral purity of the generated microwave signal could be optimized by introducing subharmonic microwave modulation via a phase modulator in the OEO loop. The experimental results show that the central frequency of the microwave signal generated by the proposed OEO could be tuned from 12 GHz to 18 GHz, and output power of the generated signal was more than 5 dBm. At the same time, the generated signal had a side-mode-suppression ratio of 51 dB and a 3 dB bandwidth of 100 kHz. Finally, the phase noise of the measured microwave signal could be optimized to -78 dBc/Hz and -109 dBc/Hz at 100 Hz and 10 kHz frequency offset by introducing subharmonic microwave modulation in the system, respectively. Furthermore, the tunable frequency range of the generated signal was restricted by the operating bandwidths of the optic-electronic devices which were utilized in the system. A higher frequency of the generated microwave signal could be achieved by using the devices with larger bandwidths in the OEO loop.