In view of the phenomenon that upright triangular zone and inverted triangular zone of the collapsed roof appeared alternately in the gob, a mechanical model of the rebreak of the key rock block (KRB) of the main roof in the gob was established by using theoretical analysis. The main reason for the rebreak of KRB was revealed and the bending moment expression at any point of KRB was obtained. So, a rebreak system of KRB of the main roof in gob was developed using Matlab software. The four influencing factors of length of KRB ( l ), rotation angle of KRB ( α ), friction coefficient ( f ), and load on KRB ( q ) were selected to study the influence on the rebreak of value and position of the maximum bending moment of KRB by using orthogonal and controlled experiment. The results show that: (1) the degree of influence on the KRB is in order: l  >  q  >  α  >  f ; (2) The maximum bending moment is positively related to l and q , is negatively related to α and is unrelated to f . (3) The positions of the maximum bending moments of KRB are all located at 0.575  l , which is not affected by l , α , f , and q . The research on the influencing factors of the rebreak of KRB can control the formation of the collapsed roof form in gob and provide a certain reference for the upward mining of coal seams between a short distance and the mining of coal seams under water bodies.