Optoelectronic oscillator (OEO) has been considered as the best technique to produce microwave signal with low phase noise. However, due to the electronic bottleneck rooted from frequency selection mechanism based on electrical filter, it is faced with such a dilemma that it is difficult to simultaneously obtain microwave source with broadband tunability and better phase noise performance. Semiconductor laser under optical injection has showed rich nonlinear dynamics and has demonstrated their potential in various domains, such as information encryption and all-optical signal processing. In this paper we propose and investigate a new modulator-free OEO scheme based on an optically injected semiconductor laser with the subharmonic modulation. The performance of the proposed OEO is optimized with respect to the single sideband phase noise, and the tunability of the generated microwave signal is discussed. A generated microwave signal at 11.42 GHz is experimentally demonstrated with the single sideband phase noise of -101.21 dBc/Hz at the offset frequency of 10 kHz.