Abstract: Abstract: With population growth, most of the grasslands in China were converted into croplands. The structure, process and function aspects of the ecosystem changed as a result from the conversion of the grassland, and led to a decline in soil quality and productivity, soil erosion acceleration, and destruction of the vegetation. These phenomena are especially obvious in karst areas, because the typical characteristics of karst areas are harsh habitats and ecological vulnerabilities. Nowadays, most investigations mainly focus on biodiversity, soil and water conservation, soil productivity, and soil carbon density. However, the studies of soil trace elements are scarce. The available trace elements of soil are very important for plant growing, especially when major elements, such as N, P, and K are rich. In order to explore the effects of the conversion of grassland into cropland on soil trace elements (Cu, Fe, Mn, Zn, B, and Mo), a paired-site design was employed between grasslands and corresponding croplands of karst area in Guizhou. At the same time, the regional indices of trace soil elements were developed based on trace soil element density. In this study, the contents of available trace soil elements in different depths were measured and the element densities were calculated for the grasslands and their counterpart croplands, and consequently the regional indices of trace soil element was built for evaluating the study area. The results showed that trace soil elements decreased with soil depth. The soil contents of available Cu, Fe, B, and Mo decreased, ranging from 4.97% to 79.17%, resulting from the conversion of grassland to cropland. The soil content of available Zn in every soil depth in cropland was higher than in grassland, increasing in range from 11.76% to 46.06%; also, the declining rate of available Mn content in cropland areas was faster than in grassland areas after the grassland conversion. As the contents of trace soil elements varied with soil depth, the previous evaluation standard, based on the trace soil element content, could not eliminate the spatial heterogeneity. Therefore, the regional indices of abundant trace soil elements, based on soil trace element densities, were developed, in which the different range of parameter values were defined as trace element's abundance degree, and the trace element density was calculated as the weight of trace elements in unit area in a certain depth. This method is proven better than previous evaluation methods, since it could avoid the side effect of soil trace element's spatial heterogeneity. According to this evaluation, the available Fe and Mn were at a very high level in the study area, the abundance of soil available Cu, Mo and B decreased, and available Zn increased due to the grassland conversion. Combining with the trace elements effectiveness evaluation index, available B in the soil was at a below medium level and could not meet the demands of plant growing. The findings provide references for developing scientific strategies of land use in fragile ecological areas, and will be helpful for developing a scientific evaluation system of soil trace elements' abundance indices.