Undeviating the evaluation of the geochemical background of an element in topsoil is very important for the correct assessment of quality of the plough. However, the difference in elemental contents between the topsoil and subsoil in the same place has been ascribed, by some authors, to artificial pollution of the topsoil. The elemental background value of the subsoil has been suggested to be the same as that of the topsoil. Analyzing the distribution characteristics of Hg, Cd, Pb and As in the fluvioaquatic soil profiles of the Taiyuan basin, and on the basis of comparing the elemental contents with that of the least polluted soil in the mountain areas,which is the source of the basin sediments, the difference content of these elements between the topsoil and subsoil in the Taiyuan basin is likely caused by natural differentiations during soil formation. In view of this, a new approach to evaluate elemental background in the topsoil is suggested. The essence of this method is that the background of an element in the topsoil is evaluated by regression analysis and/or cumulative frequency analysis but with the background value of the corresponding element in the subsoil as a reference frame. In this paper, this approach is discussed in detail and the background values of the four relevant elements in the basin topsoil are given. In addition, the elemental background value estimated by the regression analysis coincides well with that estimated by the cumulative frequency analysis.