Abstract: The Achnatherum splendens grassland evapotranspiration of oasis-desert ecotone in the northern Tianshan Mountain was calculated using Bowen ratio equilibrium energy method，then the influence of different weather conditions on the relationship between evapotranspiration and environmental factors were analysed，based on the observed meteorological data. Furthermore， evapotranspiration forecasting models were established. The results showed as follows：（1）the significance of impact of environmental factors on evapotranspiration implied that： net radiatio（Rn）> soil heat flux （G）> air temperature> air relative humidity （RH）> 5 cm soil temperature > actual vapor pressure （ea）> wind speed （V_wind）> 5 cm soil moisture. Therefore， net radiation （Rn），soil heat ?ux （G），air temperature，and relative humidity（RH） were the four major factors，whose absolute values of the correlation coefficient were above 0.44. However，except for 5 cm soil moisture （Ssoil-5 cm），all the correlation between each climate factor and evapotranspiration reachd a very significant level. （2）The indirect effect of each factor produced by Rn on evapotranspiration was greater than its direct effect，meaning that net radiation is the principal factor determining the scale of evapotranspiration. Especially，the absolute value of indirect effect coefficient of G，air temperature and [RH] produced by Rn were between 0.4 and 0.74，which were much larger than their own direct coefficients. （3）The direct negative effect of G on evapotranspiration was much smaller than other indirect positive effects，causing its comprehensive effect to be negative. For instance，the indirect effect coefficient of G produced by Rn was up to 0.744 1，which was 3.34 times as big as the direct negative effect，leading to its comprehensive effect being positive. （4）The evapotranspiration forecasting models in which evapotranspiration was chosen as dependent variable and each environmental factor as independent variable，were established with the method of multiple linear stepwise regressions. Established and certified evapotranspiration forecasting model with Rn，G，air temperature only included suggests that greater evapotranspiration of extremely dry area mainly relies on the increased intensity of the heat factor，which is different from warm and humid area relying equally on water and heat factors. The research can deepen the study of regional evapotranspiration model，which provided guidance for evaluation of regional water resources carrying capacity and desertification prevention work.

Key words: SPAC system, oasis-desert ecotone, evapotranspiration, environmental factor