Coal bump occurring in longwall roadways accounts for more than 87% of total coal bump events. However, there is no practical mechanical model that can explain the causality and process of coal bump, resulting in that the prediction and prevention of rockburst heavily depend on engineering experience or lessons. Considering the mechanical characteristics of surrounding rock, the suspended roof behind working face and stress state of the rib coal, a seesaw structure model related to incentive of the coal bump and a mechanical model related to instability of rib coal block are established in this paper based on theoretical analysis and numerical calculation, which is capable to clarify the causality mechanism, process, key factors and critical state of coal bump. The hard suspended roof behind the working face produces periodic abutment pressure on the coal around the working face. The result of numerical calculation shows that, under the condition of high compressive stiffness of hard coal around the working face, the roof in front of the working face rebounds upward. The phenomena of roof rebound suggests that the overlying strata of the working face form a seesaw structure. In the area of roof rebound, the normal stress on the roof-coal interface is reduced. Stress analysis of a rib coal block has been conducted. Result shows that, the tectonic stress perpendicular to the rib can overcome the fractional resistance pushing the rib coal into the roadway once the normal stress on the roof-coal interface de-stresses to a certain level. Accordingly, a critical state equation of coal bump has been established. It can be concluded that, de-stressing of the roof-seam interface caused by roof rebound triggers coal bump. The tectonic stress is its force source, and the tensile strength of coal and the shear strength of the coal seam with roof and floor are bump resistances. This study clarifies the causality mechanism and process of coal bump occurred in longwall roadway that provides a theoretical basis for prediction and prevention technology.