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Re-vegetation is an important practice for eco-environmental rebuilding of degraded environments. Accordingly, re-vegetation has been widely used to reduce erosion and protect soils against degradation in the Loess Plateau. However, little research has been conducted to study the effects of long-term re-vegetation on soil properties, which is essential to reveal corresponding changes in soil quality. Therefore, this study was conducted to evaluate the influence of 22 years of re-vegetation on soil quality in a semi-arid area. Soil samples were collected from a bare slope, a re-vegetated northern slope and a re-vegetated southern slope. The soil properties were then determined and the soil quality indices were calculated. The results showed that long-term re-vegetation significantly improved soil properties in the Loess Plateau, but that the effects of re-vegetation varied with slope aspect, slope position and soil depth. The greatest improvement in soil properties was observed in topsoil, on northern slopes and in lower positions of the slopes. Significant correlations were observed among soil chemical properties (including organic matter and total N contents) and soil enzymatic activities. Total nitrogen and organic matter content, activities of urease, invertase and alkaline-phosphatase could all be combined into one factor to indicate soil quality and characterize the distribution of soil properties along slopes. The resulting soil quality index (SQI) can effectively reflect the changes in soil quality in response to 22 years of re-vegetation in the Loess Plateau.