The existing stability analysis of symmetrical fixed block of roof does not consider the influence of original rock holding force and structural plane fracture

which easily leads to inaccurate analysis results. Based on the relaxation analysis method

the stress situation of symmetrical fixed block under clamping force is analyzed

an equivalent stress formula of fixed surface under the fracture of structural plane is deduced

the joint stress distribution state of fixed surface under the joint influence of clamping force and structural plane frac-ture is discussed

and the analytical solution of the minimum fixed area of symmetrical fixed block is given. The symmetrical fixed block is simulated by PFC software

the difference between the numerical solution of the minimum fixed area and the analytical solution is compared

and the field investigation is carried out on the roof breaking phenomenon of a roadway in the #105 orebody of Gaofeng mine in Guangxi. The results show that: (1) The minimum fixed area of symmetrical fixed block is positively correlated with the height of block and the angle between structural plane and vertical plane

respectively. (2) The numerical solution of the minimum fixed area is basically consistent with that of analytical solution

and the average relative error between them is 7.4%. (3) The clamped block is frac-tured and falls off

leaving visible minimum fixed area at the roof of roadway. The results are in good agreement with the results of nu-merical analysis. The research results show that the stability analysis of symmetrical fixed block of roof based on relaxation analysis method is scientific and reasonable.