Danzhou, which lies in western Hainan and where large-scale Eucalyptus plantations are present, was chosen as the target study area. We selected 1-to-5-year-old short-rotation Eucalyptus trees, 20-year-old Eucalyptus trees and 10-year-old Cocos trees with adjacent space and similar natural conditions as our experimental plots. After 3 years of continuous sampling, from 2010 to 2012, in fixed sites, we analyzed the variation of soil moisture content for Eucalyptus forests and its response to plantation ages. The results showed that: (1) Soil moisture content decreased continuously from January to April, fluctuated in May and June, increased to the annual maximum between July and October, and descended slightly during the remaining months. (2) There were significant or highly significant differences in mean monthly soil moisture content among the short-rotation Eucalyptus plantations (including a second-generation 5-year-old short-rotation Eucalyptus plantation, a third-generation 1-year-old short-rotation Eucalyptus plantation and a third-generation 4-year-old short-rotation Eucalyptus plantation), 20-year-old Eucalyptus plantation and 10-year-old Cocos plantation, except between the second-generation 5-year-old short-rotation Eucalyptus plantation and the third-generation 1-year-old short-rotation Eucalyptus plantation and between the 20-year-old Eucalyptus plantation and the 10-year-old Cocos plantation. (3) Differences in soil moisture content increased with soil depth between the short-rotation Eucalyptus plantations and 20-year-old Eucalyptus plantation, 10-year-old Cocos plantation. In the 0-30 cm soil layer, the average moisture contents for the short-rotation Eucalyptus plantations, non-short-rotation Eucalyptus plantation and Cocos plantation were 6.08%, 7.53% and 6.93%, respectively, while, in soil layers deeper than 80 cm, values were 8.10%, 11.72% and 11.95%, respectively. The phenomenon that increases in soil depth enlarge differences in soil moisture content reveals that short-rotation Eucalyptus forests consumed more water in the deep soil, compared with the Cocos forest and non-short-rotation Eucalyptus forest. (4) The variation coefficient of soil moisture content gradually declined from top to deep soil layers for the short-rotation Eucalyptus plantations, 20-year-old Eucalyptus plantation and 10-year-old Cocos plantation. In addition, the coefficient was larger for the elder short-rotation Eucalyptus forests and under deeper soil layers, which indicated that compared to the Cocos forest and non-short-rotation Eucalyptus forest, the elder short-rotation Eucalyptus forests consumed more deep soil water. (5) More continuous planting and greater stand age minified soil moisture content. The soil moisture of the 1-year-old Eucalyptus plantation was in a recovery phase and exhibited a higher content, which would improve the soil water environment and the growth of Eucalyptus trees.