Water deficit of the terrestrial topsoil is directly linked to the local climate variation. The evapotranspiration and water deficit of the topsoil over different underlying surfaces were evaluated by the data collected during the LOess Plateau land surface process field EXperiment(LOPEX), LOPEX05 and LOPEX06, which were conducted in the Chinese Loess Plateau Mesa Region. The maximal amount of evapotranspiration caused by canopy was about 0.05 mm·h^-1 at noon under the moderate soil water condition, whears the evapotranspiration could reach 4.60 mm·d^-1 in the first cloud-free day after a rainfall event over the winter wheat field, and it was 3.70 mm·d^-1 over bare soil land. Analyzed results indicate that the local precipitation was the main cause of evapotranspiration variation, whears canopy could enlarge the evapotranspiration. From the last-ten days of April to middle-ten days of July in 2006, the water deficit of topsoil were 39.9 mm·m^-2 in winter wheat field and 17.9 mm·m^-2 and 25.3 mm·m^-2 in bare soil land. The most serve soil water deficit period was from the last-ten days of May to the first-ten days of June, the maximum of ten days′ could reach 16.50 mm·m^-2. Due to the coming of raining season, the soil water began to be surplus in the first and middle\|ten days of July. From the middle-ten days of July to the last-ten days of August in 2005, water surplus of topsoil was 17.90 mm·m^-2 in underlying surface with corn and 25.30 mm·m^-2 over bare soil land. Statistical results at the different time scales indicate that the precipitation were main impact factor to evapotranspiration and determinant factor for water deficit of the terrestrial topsoil in Loess Plateau land mesa region.