Abstract: Using the initial post-buckling theory and the cusp catastrophic model,the bifurcation buckling mechanism and unstability of roof strata were studied to derive the formulae of the main roof breaking convergence during the first weighting,to build the criterion of step convergence after the main roof breaking and to determine the final step convergence value.It was pointed out that rock blocks were in a non-equilibrium state after the main roof breaking,and the step convergence was caused by the reverse rotation of single rock block and was determinately related with the breaking convergence.It was also indicated that in accordance with the occurrence behavior of roof structure,rock mass materials and thick unconsolidated layers above the roof,the integrated application of the initial post-buckling theory and the catastrophe theory could be employed to determinate catastrophic forms of the main roof structure during the first weighting.