Abstract: in order to get the accurate calculation method of the water conducted zone in the ruly mechanized top ceal caving mining in ultrathick sean under thegeological conditions of the soft and weak overburden rock strata in the east part of lner MongoliaArea,based on the No 02 cal mining fce in No. 1 Mindong Mlime ofShenbua Dayan Maining Company as a cae the cal mine water prevention and control empircal formula,caculation fomula of the coal mining under " thee surace ojects" and other method were applied to caculate the heightof the water fowing crack zone . Meanwile,the FLAC3 Desoftware was apliedto sinmulate the overburden rock water fowing crack zone development process of the seam roof overburden fallre and to analyze the waler fcwing crack zone development heightunder th diterent pushing forward dlistances. The plastic zone distrioution method was appid to obtain the wate fowing crack zone develoment height in " sudcdern" and " stable" states and the pushing forward distance of the coal mining face. With the caculatons,thvee methods showed the heights of the wate fowing crack zones would be 9.2.26.67 and 8.0o m indidually and the site trling exploration showed tht the height of the water fowing crack zone was 8.73 n.The sSte,measured data showed that the faling and mining heightratiowas 5and the fractured zone height and mining height ratio was 11 under the condition of the rll - mechanize top coal caving mining in ultra thick seam with the soft and weak overburden rock strata in the east part of Inner Mongolia Area