In coal engineering such as coal mining and roadway excavation, complex underground environment leads to damage and deformation of coal body. It is of great significance to study the response mechanism of mechanical characteristics during coal disaster damage to maintain the stability of coal mining. In this paper, the large-size coal body is taken as the research object, and the conventional triaxial compression test is carried out to monitor the mechanical behavior of coal body under different confining pressures. The conditions and laws of the sub-instability stage of coal body are found, and the failure mechanism of coal body under triaxial compression is revealed. The results show that : ( 1 ) The thresholds of the five stages of the full stress-strain curve increase with the increase of confining pressure, and the peak strain has a high correlation with confining pressure. ( 2 ) The power law equation has a good fitting effect on the axial strain rate in the accelerated growth stage of coal disaster. ( 3 ) When the test environment is close to the real state of coal underground, the sub-instability will occur ; the quasi-static and quasi-dynamic stages account for 37.5 % and 62.5 % of the total time of sub-instability, and the stress and radial strain evolution in the quasi-dynamic stage is more intense. ( 4 ) The sensitivity of residual strength is higher than that of peak strength, and the strength attenuation coefficient gradually decreases with the increase of confining pressure. ( 5 ) The coal body is mainly shear failure accompanied by tensile failure, and the crack width is different at different positions. The research results have theoretical significance for the identification of sub-instability stage, and provide theoretical support for the occurrence mechanism, early warning and prevention of coal engineering disasters.