Three-dimensional geological modeling is a common method for geological research and calculation of reserves in geology and mining industry in Western Countries, which utilizes geostatistics and the concept of regionalized variable to reasonably divide the blocks of the three-dimensional orebody model and then to estimate the grade and calculate the reserves. In this paper, with a large iron deposit in Kyrgyzstan as the research object, the authors used the correlation function between the grade and density of the ore, then translated the three-dimensional model of ore's grade valuation into ore's density (density of vertical cube). At last, by the vertical cube gravity forward calculation, a comparative analysis of the forward modeling result and measured gravity data is made. On such a basis, the authors put forward a new method for iron gravity exploration and provide a basis for increasing reserves and deploying exploration work.