In the evaluation of land suitability, dividing the principal suitability of the two-sided suitability or many-sided suitability land is very neccessary. In this paper, the principles, indices and method for dividing the principal suitability of land are discussed and a comprehensive and quantitative index——the concept of economic-social suitability index of landwhich is used for analysing the principal suitability of land is propounded. In the end, 'he Huadian Plain of the Diancang Mountains is taken as an example and its principal suitabi'ity of land is analysed. It has been found that the Plain is suitable for growing the cool-loving crops which ripen once a year.