The authors,first time in the view of logical deduction,bring out and attest equilibrium (Eq) and structural capacity (Sc),two index of Parato formula on the structure of city-size of urban system. Based upon the index, the traditional Parato formula is reconstructed as a general model,which is endowed the power to describe the size distribution of urban system,and show us all the possible situations in the development of urban system,all of which contribute much to the applying expansion by Parato formula. In the end,Parato formula reconstructed is successfully applied in the temporal and spatial study on the development of China's urban system during "the Eighth Five-year Planning".