In order to accurately predict the height of the water-flowing fractured zone of fully mechanized mining face, based on the analysis of the evolution of overlying rock structure, the mine pressure, water inflow and microseismic events, the concept of the main control roof of the water-flowing fractured zone of overlying rock is presented. Then the composite structure model with horizontal three-zone supporting body of the thick and hard overlying rock is es-tablished, and the stiffness calculation formula of the "three-zone composite supporting body" is given. The calculating formula of the critical periodic caving span of overlying rock is ob-tained, and the method to predict the height of the water-flowing fractured zone based on the main control roof is presented. The method has been successfully used in 31101 fully mecha-nized mining face in Nalinheer mine. The results show that the main control roof of the wa-ter-flowing fractured zone is the first unbroken thick hard rock from bottom to top in the overburden rock, and the main control roof is supported by the composite support body com-posed of the in-situ rock zone, the hydraulic support supporting zone and the supporting zone of the caving collapse roof. Due to swelling and evolution of relative compaction coefficient of the collapse roof, the supporting stiffness of the caving collapse roof zone takes on "slow growth" and "fast growth" two-stage characteristic. The critical periodic caving span is the square span of the working face. The measured height of the water-flowing fractured zone of 31101 working face is 17.21. The two empirical formula in “Code for the Design of Coal Pillar and the Extraction of Coal for Buildings, Water Bodies, Railways, and Main Tunnels” are em-ployed, and predicted the height of the water-flowing fractured zone are 8.51 and 9.83, respec-tively. The predicting result of the height of the water-flowing fractured zone by the method in this paper is 19.58. The predicting result from the method in this paper is closer to the measured result. The method of predicting height of the water-flowing fractured zone in this paper takes into account the influences of overburden structure, distribution of thick and hard rock, mining height, swelling characteristics of collapse rock and other factors on the height of the wa-ter-flowing fractured zone, and it provides a new way to predict the height of the water-flowing fractured zone of fully mechanized mining face.