Pregelatinized starch (PS) was used for the selective flocculation and flotation of fine siderite in a carbonate-containing iron ore. With PS, the flotation of fine siderite was improved. The repulsive forces between fine siderite particles and the attractive forces between siderite and hematite or quartz were decreased after treatment with PS, indicating that the aggregation of siderite was enhanced and the aggregations of mixed minerals were weakened. An analysis of the changes in X-ray photoelectron spectra showed that coordination bonds were formed when PS was adsorbed on siderite and hematite. However, PS could not adsorb on quartz. Moreover, the molecular simulation showed that the main mechanism for PS adsorption on siderite was confirmed as a "tail model" with end OH coordinated with Fe2+. The bridge connection of PS enhanced the flocculation of fine siderite. The flotation of fine siderite was also enhanced. For hematite treated with PS, the combination of coordination bond and hydrogen bond resulted in the "loop model" and "train model" as the main adsorption mechanisms of PS. The molecules covered the hematite surface and prevented the adsorption of the collector. The flotation of hematite was depressed. As a result, the selective flocculation and flotation of fine siderite were realized.