With less cultivated land, and sensitive to climate change, grain yield of Tibet is low and unstable, which has become an important factor to affect the social and economic development. In this study, the relationship between Tibet's grain yield and cultivated land during 1985-2010 were analyzed to find the main factors affecting grain yield in the background of climate warming, and the countermeasures and suggestions were put forward to guarantee of grain production in Tibet. Meteorological data were from 38 weather stations in Tibet, grain yield was from 2011 Tibet Automatic Yellow Book, and cultivated land area was from the remote sensing images from multi spectral scanner in 1980, thematic mapper in 1990, enhanced thematic mapper in 2000, and HJ-satellite in 2010. The four series of remote sensing data were corrected before use. In order to reveal the changes and influencing factors of grain yield, the crop climatic potential productivity was calculated by using Thornth waite model,and the crop trend production and climate production were separated by exponential curve based on the grain production and the meteorological data and four series of remote sensing data. The trend of climate change was analyzed using climatic trend rate and accumulative anomaly method. With the aid of visual interpretation and land change rate, changes of cultivated land were also discussed. The relationship between grain yield and climate change and the change of cultivated land was explored systematically using grey correlation analysis method. The results showed that:1) From 1985 to 2010 in Tibet, the average total grain production was 77.83*10~7 kg, while average climatic potential productivity and grain production per unit area was 7419.58 kg/(hm~2·a) and 1948.93 kg/hm~2 respectively; Tibet's total grain production, climatic potential productivity and grain production per unit area demonstrated a fluctuating growing trend during the period of 1985-2010; Grain production potential was achieved in an average rate of 25.86%, presenting a gradually increasing trend; Grain production was approaching climatic potential productivity and there was much space for improvement in it; 2) The ratio of climate production to grain production per unit area fluctuated between -10.74% and 8.03% and its variation range showed a decreasing trend while the influence on grain production by climate continued to decline; 3) The average accumulated temperature ≥0℃ in Tibet was 2224.53 ℃, mean temperature in growing season and precipitation changes were 11.41 ℃ and 387.48 mm, respectively, which showed an increasing trend, while average sunshine hours of growing season was 1095.45 h, which tended to decrease; Besides, the turning change of those climate factors appeared in 1994; and 4) Correlation degree between grain production and cultivated land, accumulated temperature ≥0℃ and precipitation changed from 0.633 to 1.321; Correlation degree between grain production and mean temperature in growing season and precipitation was 0.595 and 0.641. Cultivated land, accumulated temperature ≥0℃ and precipitation in growing season were the most important factors affecting food production. The research can provide valuble information for identification of grain yield and its influencing factors, and to further improve the grain yield and to ensure the sustainable development of social economy in Tibet.