Abstract: A long term tendency forecast method is introduced.With the forecaster's experiences and statistic,divisioned study of multi-layered monthly mean grid data of 500 and 100hPa geopotential height and Pacific SST based on changes in pattern characteristics,computation is done by the theory of similar discrete value.It is the purpose of the method to improve on the one-sided emphasis arisen from similarity comparison by both taking into account information on multiple spatial levels and involving comprehension in terms of dynamic similaring for its change over a period as long as a number of months.It proves satisfactory in forecasting experiment with long-term tendency of precipitation and temperature in Guangdong.