Abstract: In order to deeply analyze the characteristics of crack development and strain evolution of coal samples under uniaxial compression, digital speckle technology was used to obtain the full-process images and data of the crack field and strain field of the specimen during the test. The global cracks development and strain evolution process of the specimen are analyzed, and the internal relationship between strain evolution and crack development is obtained. Based on this, the key cracks of the specimen are selected, the development of local cracks is further analyzed, and the evolution characteristics of the local horizontal strain, vertical strain, maximum principal strain field and the direction of the maximum principal strain with the acquisition frame are clarified. The results show that: the strain evolution of the specimen is potentially related to the development of cracks. There is often obvious strain concentration in the development of cracks, and there will be concentrated distribution of horizontal strain and vertical strain at the inclined or curved cracks. And the horizontal strain concentration is relatively large. In a curved crack, When the degree of opening is large, it is affected by the vertical compressive stress, which tends to produce large vertical deformation, and induce horizontal deformation in the opposite direction. There will be phenomena such as crack closure, sharp increase in vertical strain, and sharp decrease in horizontal strain. When the crack opens or expands, the maximum principal strain increases, and at the same time, there will be obvious strain concentration; When the crack is closed, the maximum principal strain decreases and the strain concentration also decreases; The direction of maximum principal strain is basically the same as the crack opening direction, and is basically perpendicular to the crack propagation direction. In addition, when the acquisition frame reaches 89% of the failure frame of specimen, the specimen's cracks develop rapidly, and thereafter, it also shows rapid concentration of local strains. And when the specimen cracks are generated or destroyed instantaneously, the maximum principal strain direction will also change greatly. The test results can be used as one of the precursors of the specimen failure early warning.