Abstract: Mining under water needs to observe height of water-flowing fractured zone, to determine a reasonable upper mining limit. The surface of excavating face H2106 in Longkoubeizao mine is sea. To solve the technical problems in drilling holes for observing height of the water-flowing fractured zone, the height of the water-flowing fractured zone was obseved by using upward holes and detector for observing height of the water-flowing fractured zone. This method requires small amount of drilling works and has high observation accuracy. In excavating face H2106 three observation section were set, 4 pre-mining holes and 8 after-mining holes were drilled for observing height of the water-flowing fractured zone,and the upper limit of water flowing fractured zone was determinated by using the water content change of marl aquifer in the roof before and after mining . By observing,the height of water flowing fractured zone of excavating face H2106 was 38.8 m, the ratio of height of the water-flowing fractured zone and mining thickness was 9.5.