Abstract: In order to solve difficult support problems of the soft and broken large cross section open-off cut under the watery roof condition in the Mine of Datong, a theoretical analysis, experiment simulation and site observation means were applied to reveal the main support failure causes of the large cross section open-off cut with watery roof and broken surrounding rock.A step by step combined support, roof water drainage, fixed point grouting reinforcement and water sealing coordinated te chnology were applied to the site observation on the stability conditions of the open-off cut.The results showed that roof water drainage with boreholes was about 25 00 0 m3 and the roof water affected to the surrounding water and supports of the gateway was effectively overcome.The subsidence value of the roof in the sections of the once completed driving gateway was controlled within 400 mm.The subsidence value of the roof in the sections of the secondary enlarged sidewalls was controlled withi n 250 mm.