We numerically investigate high-frequency microwave signal generation utilizing a double injection locking technique. A slave laser (SL) is strongly injected by a master laser 1 (ML1) and a master laser 2 (ML2) optically. Stable locking states are observed when the SL is subject to optical injection by either the ML1 or the ML2 individually. By utilizing the hybrid scheme consists of double optical injections, the advantages of each individual dynamical system are added and enhanced. Comparison of the performances of the spectral width, power fluctuation, and frequency tunability between the signal generated in the double injection locking scheme and the similar period-one (P1) oscillation signal generated in a conventional single injection scheme is studied. A 3-fold linewidth reduction is achieved by utilizing the double injection locking scheme benefitted by the strong phase-locking and high coherence when operating at the stable injection locking state. Moreover, for the double injection locking scheme, a wide continuous tuning range of more than 100 GHz is obtained by adjusting the detuning frequency of the two master lasers. The performances of narrow linewidth, wide tuning range, and frequency continuity show the great advantages of the high-frequency microwave signal generated by the double injection locking technique.