Abstract: At first, site classifications and their fuzzy characteristics was analyzed. On the basis of a comprehensive consideration of the classification of site in characters of the current earthquake resistant codes, two dimensional membership function of 4 types of site was constructed and the fuzzifying of the site classification was realized. Then, three fuzzifying site classification methods were provided, the continuity of the site classification was actualized, and the application of various methods to seismic design of structure was disussed. Finally, the continuous method of the site characteristic period and the two-dimensional graphs were provided. It is shown that the characteristic period distribution confirmed by this method corresponds with the equivalent chart of the code for seismic design of buildings. This method gets rid of the illogical discontinuity and appreciable distinction displayed on the code chart between the site edges, basically realizes the unification of the site classification between code of seismic design of buildings and special structures, and lays the foundation for fully considering the effect of site condition in earthquake engineering design, objectively estimating earthquake action and adopting reasonable earthquake protective countermeasures.