In this paper, we propose and numerically investigate a novel scheme to optically generate microwave signal based on mutual injection locking. A twin-stripe semiconductor laser is driven into mutual injection locking state with two phase-locked wavelengths. Microwave signals with low phase noise are achieved. The frequency of the demonstrated microwave signals can be tuned by adjusting detuning frequency between the twin lasers. The results show that our proposed scheme is featured with not only lower phase noise but also better noise tolerance compared with typical master-slave injection configuration.