Abstract: This paper pointed out the shortage of equivalent frame method to design unbonded prestressed concrete flat-floors.By analyzing the moment distribution of such a structure,it was indicated that the design of the serviceability limit state should be based on the elastic moment in order to control the crack width,and as to the ultimate state,the internal force redistribution should be taken into consideration while the crack width under service load was guaranteed.Based on the above mentioned principles and some calculations,the paper concluded the magnitude of internal force redistribution and discussed the moment distribution after modification.Finally several design suggestions were put forward.