The performances of microwave signals generated by an optically injected semiconductor laser operated at the period-one (P1) oscillation under 1/2 subharmonic microwave modulation, are investigated experimentally. The experimental results show that under suitable injection condition, the microwave signal output from an optically injected semiconductor operated at P1 oscillation can reach a frequency of 26.5 GHz limited to the experimental conditions and may have a single sideband optical spectrum structure, but the linewidth of the microwave signal is relatively wide (on the order of MHz). After adopting 1/2 subharmonic locking technique, the linewidth of the obtained microwave signal can be reduced from tens of MHz to tens of kHz. Furthermore, we analyze the influences of the power and frequency of the subharmonic microwave on the phase noise of the generated microwave signals, and further map the subharmonic microwave locking region in the parameter space of the power and frequency of the subharmonic microwave.