Taking the Jialu River Basin of the Loess Plateau as the study area, this study collected the hydrometeorological data from 1966 to 2006, meanwhile selected 60 flood data to analyze the flood process line, runoff characteristic value, and runoff generation mechanism. Combined with the changes of forest and grass vegetation in Jialu River Basin in the past 30 years, the response relationship between vegetation coverage and watershed runoff mechanism was analyzed comprehensively, which revealed the reason for the dramatical water and sediment reduction in the Yellow River Basin. The results showed that in the past 30 years, the volume of runoff and sediment in the Jialu River Basin decreased sharply. Compared with the 1960s, the annual average volume of runoff was abated by about 75.0%, the vegetation coverage rate increased from 8.0% to 35.0% and the watershed storage capacity of the basin had the same growth trend with the vegetation coverage. The proportions of floods dominated by runoff formation in excess of infiltration in the three stages(1966-1975,1976-1996,1997-2006) was 72.8%, 65.0% and 60.0%, respectively; however, compared with the period of 1966 to 1975, the proportions of floods dominated by runoff formation at the natural storage were increased by 8.4%, 4.3% respectively during 1997 to 2006. The research results would help to realize the runoff law of the main water and soil loss areas in the Yellow River Basin, reveal the mechanism of water changes in the Yellow River, and provide a scientific basis for ecological construction in the area.