Soil erosion and soil quality degradation following deforestation are major processes responsible for eco-environmental deterioration on the loess Plateau. However, there is little information available on soil quality degradation caused by deforestation and increased soil erosion. To quantify the degradation rates, we selected eight croplands on southern slopes with different years of cultivation within 100 years in the Ziwuling area as a case study. Using correlation analysis in conjunction with effectiveness, completeness, and sensitivity of all 20 indictors, we selected eight indictors, i.e., soil organic carbon, fungi amount, alkaline phosphatase activity, invertase activity, capillary porosity, coarse clay, coarse silt, and mean weight diameter of water-stable aggregate(MWD) for use in the evaluation system. A quantitative Soil Quality Index (SQI) comprised of these 8 indicators was developed using a principal component analysis. Results showed that SQI in the 0-20 and 20-40 cm soil depths of the forestlands were 0.909 and 0.427. The SQI decreased in the top 40 cm layer as cultivation years increased, especially during the first four years of cultivation with annual reduction rates of 0.073 and 0.028 in the first (0-20 cm) and second (20-40 cm) depths respectively. Compared with the corresponding forestlands, SQI during the initial four years of cultivation decreased by 26.2% and 32.3% in the 1st and 2nd depths, respectively. After 12 years of cultivation, SQI in the 1st and 2nd depths were 0.594 and 0.235, decreased by 39.6% and 45.0%, respectively. Reduction rates in the 1st and 2nd depths during 4-12 years of cultivation were 10.9% and 35.7% relative to the initial four years of cultivation. After 43 years of cultivation, SQI in the 1st and 2nd depths were 0.333 and 0144 and reduced by 64.4% and 66.3%, respectively, compared with the forestlands. Reduction rates in the 1st and 2nd depths during 12-43 years of cultivation were 9.6% and 10.7% relative to the initial four years of cultivation. During 100 years of cultivation, SQI in the two depths were 0.140 and 0.068 and reduced by 84.6% and 84.10%, respectively. Reduction rates in the two depths during 100 years of cultivation were 4.1% and 3.6% relative to the initial four years of cultivation. Meanwhile, the reduction rates of SQI in the 1st depth were greater than those in the 2nd depth, and the differences of SQI between the two layers decreased as the cultivation years increased, which implied that accelerated erosion could be a main cause for soil quality degradation in the region.