Based on four tree-ring chronologies which have been analysed with appropriate collec-tion and accurate dating in four sites of the middle Xizang plateau, an essential process on re-construction of past climate has been pointed out in this paper. First, the response functionof each dendrochronology has been built and used to estirnate how ring-width growth res-ponds to variations in monthly climatic change. The response values to air temperature andrainfall appear significant and reliable results, and the multiple correlation coefficients of theresponse regression for three dendrochronologies are more than 0.85. Second, the climate fac-tors which could be reconstructed with different tree--ring series have been se1ected. The fourclimate predicands are precipitation in growing season, temperature in Spring, annual tem-peraturc and annual precipitation, respectively, for four tree-ring series. Then, the transferfunction equation, including a new set of orthogonal variables, can be used to reconstruct localpast temperature or precipitation. The orthogonal translation is adopted by Emperical Or-thogonal Function (EOF). It should be emphasized that prior growth has been considered inthe relationship between climate factor and tree-ring chronology. That means the prior growth,the lag one through three years, has been put into the final f regression equation as indepen-dent factors. Besides, some different periods for calibration,and verification have been divi-ded. Some statistics and other kind of proxy data have been adopted as test approaches. Al1statistical and other--data test has proved that the reconstruction of past climate using tree-ringdata in the middle Xizang plateau is reasonable and reliable. The basic procedure could beapplied to dendroclimatic analysis in other areas.