The northeast black soil region, an important grain production area in China, is suffering from serious productivity degradation since long term conventional cultivation. It is helpful to develop an easy and practical productivity assessment method for black soil resources conservation. Soil samples were taken and analyzed for selected fields in the Hebei watershed, located in the north part of typical black soil region of Northeastern China. The quantitative relationship between top soil thickness and soil productivity was established and applied to assess soil productivity. Results showed that there was a logarithmic relationship between top soil thickness and soil productivity, which increased with increasing soil depth. However, the increasing rate varied and the rate was faster for soils at depth less than 40-50 cm than that at depth greater than 40-50 cm. Both long-term soybean yield and local land quality classing certified the rationality of the method, especially for assessing the spatial variation of soil productivity. In the studied watershed, tillage had significantly effects on the spatial distribution of soil productivity, which increased from slope top to toe generally. However, little variation existed within the field for contour cultivated fields, but obvious difference existed for up-down slope cultivated fields. It is necessary to practice conservation tillage.