Abstract: When evaluating the heavy metal content of soils and crops in the Dexing copper mine by using a single exceeded index, the authors found that excessive heavy metals in soil and in crop are not inconsistent with each other. Pb and Cd are not or rarely excessive in soil, but they seriously exceed the set standard in crops. In order to solve this mismatch phenomenon, the authors, on the basis of National Food Hygiene Standards，proposed the method of soil heavy metals risk threshold based on crop safety and determined the risk threshold in farmland soils of the Dexing copper mine. This method overcomes the shortcomings of the deviation of the standard-surpassing boundary and the unitary soil consideration during the evaluation based on National Standards and hence can be more flexibly used in agricultural production.