Based on the principle of multi-mode interference of multimode fiber and the algorithm of image processing, the characteristics of gray level co-occurrence matrix of phase spectrum of multi-mode fiber output speckle pattern were extracted, a new speckle pattern processing algorithm was proposed and applied to micro-displacement sensing. The relationship between the displacement and the characteristics of the phase spectrum of the output speckle pattern was analyzed in detail, when the single-mode-multimode fiber structure based on step-index multimode fiber generated micro-displacement in the radial direction of the two optical fibers. The analysis of speckle patterns of various structural parameters shows that, the approximation linear relationship between radial micro-displacement of the two optical fibers and the two characteristics including correlation and homogeneity can be obtained. Therefore, in order to achieve micro-displacement measurement, the method of image processing algorithm over speckle pattern can be used, making further efforts to realize optical fiber sensing. The experimental results show that compared with the commonly used algorithm of calculating normalized intensity inner product of the speckle pattern, when the core diameter of multi-mode fiber is small, the linearity and dynamic range are relatively close, when the core diameter is larger, the dynamic range is doubled while the linearity is improved. Therefore, the proposed algorithm has better stability and wider application range.