The ribs palling was one of the main factors restricting the safe and high efficient mining of fully-mechanized caving with great mining height. Research showed that the place where rib spalling likely to take place was the middle-up area of coal wall, but, the support force of flipper was ignored and the analysis of the results needed to be discussed. Based on these new established calculated model, further more the control effect of flipper moment between support strength and length were calculated and discussed, which provided reliable basis for parameter selection under certain deflection.