A series of imitating tannin extract desulfurization solution that contained by-products of different concentration was prepared according to the accumulation process of by-products in desulfurization solution. The effect of by-products in desulfurization solution of tannin extract on desulfurization solution density was investigated. The conclusion was that the accumulation of by-products in tannin extract desulfurization solution made desulfurization solution density increase and the main factors which influenced tannin extract desulfurization solution density were Na2S2O3and NaSCN. Relation between desulfurization solution density and main by-products concentration was discussed. A good linear relationship was obtained through the regression analysis of experimental data by excel. Theory reference was offered to estimate density of tannin extract desulfurization solution in actual production.