An investigation and assessment of pollution of thallium in soils around a thalliumcontaining slag pile site by enrichment factor(EF) and geoaccumulation index(Igeo) was carried out in a sulphuric acid plant in Guangdong province. It is found that thallium pollutants were mainly concentrated in soil surface layer(0~16.5cm) and shown characteristics of contents rapidly decreasing along the uprights. And the adsorption capability of Tl is not saturation in surface soil layer, then the thallium pollutants will accumulate more and more in future. It is concluded that by Igeo method for determination, the degree of Tl contamination was more serious in exterior profile than inner profile under slags. Tl in soils underneath slag about 0~6cm, 8.5~10.5cm and lower levels(>15cm) characterized as middle, without—slight, without contamination respectively; in exterior soil about 0~1cm, 2~10.5cm, deeper soil(>15cm) showed strong, middle and without contamination. The result of EF is similar to Igeo result .The methods of EF and Igeo to assessment the Tl pollution in soil are acceptable, but these two methods can not interpret the phases, migration and bioavailability of Tl in soil.