Causes of variation of earth-air temperature difference(Ts-Ta) in arid regions of Northwest China were analyzed based on the monthly surface temperature, air temperature, precipitation and the wind speed data from 49 weather stations and NCEP/NCAR monthly reanalysis data from 1961 to 2000. The results showed that value of Ts-Ta exhibited a descent tendency in winter during the last 40 years of the 20th century. The reason is the global warming after the 1980s, increasing of height field and temperature, appearing of anticyclone circulation anomaly, decreasing of cold wave activities and wind speed at the surface layer all caused more increase of the air temperature than earth temperature. In early spring, the height field in middle troposphere reduced, and air temperature and precipitation increased, and cyclone circulation anomaly appeared in wind fields, and the wind speed at surface layer decreased to air temperature increased less than surface temperature, so Ts-Ta exhibited a escalation tendency.