In China we are able to study historical climate and it’s change by means of climatic information from historical records. Bacause there remain some deficiencies such as description, discontinuity and inhomogeneities, neither can a conparison be made between different historical periods nor can the data processing be conducted by computer. Therefore their use value in scientific research has been limited considerably.The information processing of historical climate relies on the basis of the information. The purpose is to establish a set of methods to turn the qualitative information on historical climate into parameters of climatic change, to eliminate the inhomogeneites of various sources, so that to establish a pattern of historical climate.In this paper, a set of methods of establishing the grade patterns of historical floods and droughts during the last 863 years in the Tai Hu basin on the basis of collecting systematically historica events is put forword as follows:1. According to the relationship of drought and flood conditions between each station and the region, the information sources of establishing a grade pattern of historical floods and droughts can be determined and can be further classified into three types: main type, subsidiary type and reference type, so as to establish an information network.2. Defining the classification criterion of historical floods and droughts, the grades of floods and drought may be decided, according to the information on historical floods and droughts at each station; and the corrective parameter of the grades settled, according to the 昪haracteristics of topography and climate.3. According to the differentiation of information sources,the author determines respectively the following information intensities:1) Those including exact records of floods droughts are called "strong information";2) Those which have no exact records of floods or droughts, yet can provide information through other records are called "weak information";3) Those which have no records are called "assumably normal year" or "extremely weak information".On the basis of the above cliassification the information reliability can be determined according to the information intensity.4. The regional grades are synthesized in accordance with their type by historical drought and flood grades of each station, using the method of mean with varied weight. Then the basin grade is synthesized by regional grades, using the method of establishing a distinctive model, to set up the pattern.The grade series is built on the basis of establishing an information management system.