Abstract: Based on the measured data of support column expansion at No． 44305 working face in Yujialiang Coal Mine,and compared with the support resistance monitoring data,the paper studied the dynamic interaction between support and roof from the characteristics of roof sag (period sag and cycle sag) and yield ability to the pressure of sup- port. The results showed that the coal cutting made a stable supporting system which formed by direct roof and support unbalanced,then after moving support,caused a roof circular sag and support resistance increase before a new stable supporting system formed. Roof period sag was accumulated by cycle sag in its movement period. The roof period sag at No. 44305 working face was 200-300 mm,maximum up to 462 mm;sinking speed was 40 -60 mm / h,cycle sag was nearly about 30 mm,maximum up to 52 mm. The column expansion allowance was an important evaluation indicator to the yield ability to the pressure of support. The lowest mining height should be controlled in order to ensure the support column expansion allowance to be kept at 300 mm at least,avoiding long time lower support height working condition and reducing the risk of roof cutting.