A standard commercialized fiber-pigtailed distributed feedback (DFB) laser is chosen as the slave laser to construct the master-salve laser injection locking experimental system. The basic characteristics of the slave laser at different injection powers are measured and compared, such as the stable locking range, mode spectra, phase noises and output optical powers, which prove that the laser injection locking can greatly improve the spectral purity of the slave laser while almost not affect its output optical power. The locked slave laser acts like a narrow tunable optical filter and affords some power amplification as well. Moreover, the central frequency and bandwidth of the filter are completely decided by the input injection. Then, the DFB laser injection locking technology is used to extract and amplify the single frequency of Brillouin shift in Brillouin optical fiber sensing system based on electro-optic microwave modulation. Furthermore, the drift of the direct current operation point is resolved as well.