It is well-known that Cd concentration in the soil, Cd bioavailability, and Cd bioaccumulation in rice (Oryza sativa L.) grains vary greatly in different soil parent materials. Therefore, the classification of agricultural land environmental quality and agricultural land safe usage based on the Cd content in the soil has a wide deviation. By the application of systematic sampling and analysis of soil and rice samples, and in comparison with the different soil parent materials in Heng County, it was found that soil derived from carbonate rocks has higher total Cd and lower mobile Cd proportion than soil from the non-karst areas. This result indicated that soil carbonates raised soil pH and the adsorption of Fe/Mn oxide/hydroxide on Cd significantly reduced the bioavailability of Cd in karst areas. In contrast, acidic soils with relatively lower CaO, TFe₂O₃, Mn and total Cd contents, the grown rice plants accumulated higher Cd in their grains. Further research confirmed significant differences in Cd bioaccumulation abilities in the soil between karst and non-karst areas. On this basis, the bioaccumulation factor prediction models of Cd in rice grains were developed in karst and non-karst areas. According to the total concentration of Cd in topsoil samples obtained from the area survey and the predicted Cd content in rice grains, an agricultural land safe usage scheme was put forward. The results showed that the agricultural land classification method based on Cd concentration in the soil and rice grains was more accurate and scientific than that based on the Cd contents in the soil alone.