In order to study depth of floor failure zone after mining 14311 fully mechanized caving face, three methods of mechanical model formula, empirical formula and numerical simulation were used. The floor failure depth were 8.7 m, 8.3m, 24.44 m, 22.67 m and 21.72 m respectively through calculation of  formulas in application procedures,the maximum was 24.44m. The floor failure depth were 12m and 15m respectively through numerical simulation. Combining results from comprehensive theoretical calculation and numerical simulation, the maximum possible failure depth was 24.44m.