Mutual injection locking is recognized as one of the most important and promising research directions for the fibre laser combining technology. Basing on the mutual injection locking of the two fiber lasers, the far field and the spectra of the two fiber lasers were inspected by infrared CCD and optical spectrum analyzer when the spectrum of one fiber laser and the coupling coefficient were changed. From the experiment, we conclude that the stronger was the mutual injection locking when the closer the spectra of the two fiber laser are; and the mutual injection locking would fail when the difference in the frequencies exceeds the locking range, which can be extended by increasing the coupling coefficient of the fiber lasers.